Add a server endpoint for serving devtools extensions

[kenzieschmoll]

This PR

• moves extension_model.dart to packages/devtools_shared so that we can reuse this model from the devtools server
• creates ExtensionManager that manages moving devtools extension assets to build/devtools_extensions so that the server can serve them (implemented in https://dart-review.googlesource.com/c/sdk/+/305043)
• adds a server endpoint to serve devtools extensions

work towards #1632

[bkonyi]

First pass.

[bkonyi]

Just confirming, this is a straight move from package:devtools_app/src/extensions/embedded/extension_model.dart right?

[kenzieschmoll]

I added dart doc to the members and some validation in the factory constructor so it should be re-reviewed. I also made the json parsing for the icon code point able to handle both strings and ints

[bkonyi]

Another pass.

[bkonyi]

If config is always expected to be Map, why don't we restrict it to Map<Object, Object?> in Extension?

[kenzieschmoll]

See discussion at dart-lang/tools#129 (comment)

[bkonyi]

LGTM with one last suggestion.

[bkonyi]

We should use pattern matching here for JSON validation:

  if (json
      case {
        nameKey: String name,
        pathKey: String path,
        issueTrackerKey: String issueTracker,
        versionKey: String version,
      }) {
    return DevToolsExtensionConfig._(
      name: name,
      path: path,
      issueTrackerLink: issueTrackerLink,
      version: version,
      materialIconCodePoint: codePoint,
    );
  }
  throw StateError(
    'missing required fields ${nullFields.toString()} in the extension '
    'config.json',
  );

[kenzieschmoll]

I will address this comment as part of #6097.