feat: allow producing messages to a Kafka topic from an editor or message viewer document

[shouples]

Summary of Changes

Closes #764.

Instead of showing the open-file dialog for producing a message to a topic, we're now showing the same "URI quickpick" that we use for schema upload actions, which means any .json files/documents or JSON-language editors can be used to produce a message:

So a user can now effectively re-produce a message that was opened as a message viewer preview (#799), or one they were working on in a new editor (but didn't save to a file on disk).

Any additional details or context that should be provided?

Pull request checklist

Please check if your PR fulfills the following (if applicable):

Tests

• Added new
• Updated existing
• Deleted existing

Other

• All new disposables (event listeners, views, channels, etc.) collected as for eventual cleanup?

• Does anything in this PR need to be mentioned in the user-facing CHANGELOG or README?
• Have you validated this change locally by packaging and installing the extension .vsix file?

  gulp clicktest