## Summary of the Pull Request

Adds a visible indicator that a Terminal window is elevated. This icon can be disabled with `"showAdminShield" false` in the global settings.

## References

* spec'd in #8455 
* Also in https://github.com/microsoft/terminal/projects/5
* big picture: #5000

## PR Checklist
* [x] Closes #1939
* [x] I work here
* [n/a] Tests added/passed
* [ ] Requires documentation to be updated - yea probably

## Validation Steps Performed

![image](https://user-images.githubusercontent.com/18356694/133293009-4215e319-fbf9-4ca8-8af5-afe2fa8bb62d.png)

![image](https://user-images.githubusercontent.com/18356694/133292970-90cb17fd-16c7-429a-a25f-8457850eb278.png)

## Summary of the Pull Request

This is the resurrection of #8514 and #11310. WE determined that we didn't want to do #11308 after all, so this should be profile auto-elevation, without the warning.

This PR adds two features:
* the `elevate: bool` property to profiles
  - If the user is running unelevated, **and** `elevate` is set to `true`, then instead of opening a new tab, we'll open an elevated Terminal window with the profile.
  - Otherwise, we'll just open a new tab in the existing window. This includes cases where the window is elevated, and the profile is set to `elevate:false`. `elevate:false` basically just means "do nothing special with me".
* the `elevate: bool?` property to `NewTerminalArgs` (`newTab`, `splitPane`)
  - This allows a user to create an action that will elevate the profile, even if the profile is not otherwise set to auto-elevate.
  - `elevate:null` (_the default_) does not change the profile's elevation status. The action will use whatever is set by the profile.
  - `elevate:true` will attempt to auto-elevate the profile
  - `elevate:false` will do nothing special. 


## References
* #5000 for obvious reasons
* Spec'd in #8455

## PR Checklist
* [x] Closes #632
* [x] I work here
* [x] Tests added/passed
* [ ] Requires documentation to be updated - sure does, but that'll come all at the end.

## Detailed Description of the Pull Request / Additional comments

After playing with de-elevation a bit, it turns out it behaves weirdly with packaged applications. I can try and ask `explorer.exe` to launch the process on our behalf. However, if the thing we're launching is an execution alias (`wt.exe`), and we're elevated, then the child process will _still launch elevated_. 

There's also something super BODGEY at work here. `ShellExecute` is the function we use to ask the OS to elevate something for us. But `ShellExecute` needs to be able to send a window message to the process that called it (if the caller was a WINDOWS subsystem application). So if we die immediately after calling `ShellExecute`, then the elevated process never actually spawns - sad. So we're adding a helper process, `elevate-shim.exe`, that lives in our process. That'll be the one that actually calls `ShellExecute`, so that it can live for the duration of the UAC prompt.

## Validation Steps Performed

* Ran tests
* Opened a bunch of terminal tabs at various different elevation levels
* opened new splits too
* In the defaults (base layer) as well, for madness 

Some settings to use for testing

<details>

```jsonc
    "keybindings" :
    [
        ////////// ELEVATION ///////////////
        { "keys": "f1", "name": "ELEVATED TAB", "icon": "\uEA18", "command": { "action": "newTab", "elevate": true } },
        { "keys": "f2", "name": "ELEVATED, Color", "icon": "\uEA18", "command": {
            "action": "newTab", "elevate": true, "commandline": "PowerShell.exe", "startingDirectory": "C:\\Windows", "tabColor": "#bbaa00"
        } },
        { "keys": "f3", "name": "unelevated ELEVATED", "icon": "🙃", "command": {
            "action": "newTab", "elevate": false, "profile": "elevated cmd"
        } },
        //////////////////////////////
    ],

    "profiles":
    {
        "defaults":
        {
            "elevate": true,
        },
        "list":
        [
            {
                "hidden":false,
                "name" : "cmd",
                "commandline" : "cmd.exe",
                "guid": "{0caa0dad-35be-5f56-a8ff-afceeeaa6101}",
                "startingDirectory" : "%USERPROFILE%",
                "opacity" : 20
            },
            {
                "name" : "the COOLER cmd",
                "commandline" : "c:\\windows\\system32\\cmd.exe",
                "startingDirectory" : "%USERPROFILE%",
            },
            {
                "name" : "the sneaky cmd",
                "commandline" : "c:\\windows\\system32\\cmd.exe /k echo sneaky sneaks",
                "startingDirectory" : "%USERPROFILE%",
            },
            {
                "name": "elevated cmd",
                "commandline": "cmd.exe /k echo This profile is always elevated",
                "startingDirectory" : "well this is garbage",

                "elevate": true,
                "background": "#9C1C0C",
                "tabColor": "#9C1C0C",
                "colorScheme": "Desert"
            },
            {
                "name": "unelevated cmd",
                "commandline": "cmd.exe /k echo This profile is just as elevated as you started with",
                "elevate": false,
                "background": "#1C0C9C",
                "tabColor": "#1C0C9C",
                "colorScheme": "DotGov",
                "useAcrylic": true
            },
        ]
```

</details>

Also try:
* `wtd nt -p "elevated cmd" ; sp -p "elevated cmd"`
* `wtd nt -p "elevated cmd" ; nt -p "elevated cmd"`




This was merged manually via 

```
git diff dev/migrie/f/non-terminal-content-elevation-warning dev/migrie/f/632-on-warning-dialog > ..\632.patch
git apply ..\632.patch --ignore-whitespace --reject
```