Add option for comparing NuGet structure and NuGet metadata.

[jpobst]

Currently, we use api-tools in AndroidX and GooglePlayServices repositories to help us verify changes to C# APIs when we make binding changes.

However, there are some additional scenarios we would like to verify changes to:

• Changes to the NuGet metadata, like <title> or <description>
• Changes to the files contained in the NuGet package, like to ensure that a needed .targets file doesn't disappear
  • Note this is only comparing which files exist in the package, not if the file contents changed

These additional checks will help give us more confidence to update our project file templates without silently breaking packages.

Implemented as a new option:

api-tools nuget-diff --compare-nuget-structure

If there are changes to NuGet metadata or contents, this will produce a file called nuget-diff.md in the output directory. This file looks like:

## Xamarin.AndroidX.Window

### Changed Metadata

```
- <title>.NET for Android (formerly Xamarin.Android) AndroidX - window</title>
+ <title>.NET for Android AndroidX - window</title>
```

### Added/Removed File(s)

```
- lib/monoandroid12.0/Xamarin.AndroidX.Window.pdb
- lib/net6.0-android31.0/Xamarin.AndroidX.Window.pdb
```

[mattleibow]

/azp run

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