Re-implement the event generator as a new-style plugin

[akhenry]

Summary

The Event Generator plugin provides an example of a telemetry source that produces strings. It was implemented using the legacy API (including the old type API, telemetry API, etc.).

The legacy API has not been used for new development for years and support will soon be removed completely. We want to maintain this useful example code, so we should migrate it to a plugin that defines the event type using the modern type api, as well as using the modern telemetry API.

A note on migrating Types

Types registered using the legacy bundle registration like so:

"types": [
  {
      "key": "folder",
      "name": "Folder",
      "cssClass": "icon-folder",
      "features": "creation",
      "description": "Create folders to organize other objects or links to objects.",
      "priority": 1000,
      "model": {
          "composition": []
      }
  }
]

Should be migrated to the types API:

  openmct.types.addType('folder', {
      name: "Folder",
      key: "folder",
      description: "Create folders to organize other objects or links to objects without the ability to edit it's properties.",
      cssClass: "icon-folder",
      creatable: true,
      initialize: function (domainObject) {
          domainObject.composition = [];
      }
  });

Code that was retrieving type definitions using the legacy TypeService like so:

typeService.getType(domainObject.type)

Should now use the Open MCT type API:

this.openmct.type.get(domainObject.type)

[akhenry]

@scottbell Would you mind taking a look at this?

[scottbell]

@scottbell Would you mind taking a look at this?

Wilco!

[scottbell]

@akhenry this looks pretty straightforward. One question though: do you want me to integrate this in with the other generators, or keep it standalone?

[scottbell]

Talked with @akhenry about above 👆, will keep it separate.

[scottbell]

FYI, I've got a working branch here:
https://github.com/nasa/openmct/compare/master...scottbell:mct4552?expand=1

[scottbell]

Testing Instructions

• Ensure you can still create an Event Message Generator
• Ensure you receive event data with Local Clock & Fixed time spans
• Ensure you can drop the Event Message Generator onto a Telemetry Table

[scottbell]

I've opened a PR for review here:
#4620

[akhenry]

Verified that the implementation works as specced.

Have filed a followup issue to address a low priority bug - #4879