Refactor resource loading

[jlrobins]

Summary of Changes

• Unified the loading of common CCloud resources backing the Resources and Topics panels, improving
  performance and consistency, issue #147
• Did so via new CCLoudResourcePreloader singleton class
  • Listens for ccloud login events. When connected, immediately starts populating the non-topic-level ccloud resources.
  • Method ensureResourcesLoaded() allows either the Resources or Topics view controllers to block on the loading having been completed prior to consulting the ResourceManager cache for those elements. If the loading has been completed already, will not block.
  • Fixes CCloud topic schemas don't load unless the Resources view loads environments #147 , might possibly also improve Confluent Cloud Resources do not load #224 .

Any additional details or context that should be provided?

All tests passing locally.

Pull request checklist

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

Tests

• Added new
• Updated existing
• Deleted existing

Other

• 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