Unskip loadtopicschemas test and many improvements along the way

[jlrobins]

Summary of Changes

• ResourceLoader changes:
  • New APIs and implementations for local and ccloud:
    • getSchemaRegistryForCluster(): Get the schema registry appropriate for the Kafka cluster. Born out of previously private implementations.
    • getSchemasForCluster(): Get the schema bindings appropriate for the Kafka cluster. These were previously coded as lambdas within the getTopicsForCluster() implementation.
    • getSchemasForTopicRegistry(): Get the schema bindings appropriate for the kafka cluster holding the topic.
  • Improve performance of getTopicsForCluster() implementations by refactoring and (possibly) fetching the topics and schemas concurrently, then matching them in separate code block.
  • CCloudResourceLoader.getInstance() typed to return CCloudResourceLoader instead of the base class ResourceLoader.
  • Improve docstrings.
• topicQuickPick(): load schemas and topics concurrently using loader APIs.
• Fix skipped test "loadTopicSchemas() should return schemas for CCloud Kafka topics when available", needed some additional setup at the ResourceLoader level.
• Implement loadTopicSchemas() atop loader.getSchemasForTopicRegistry().
• Delete now unused getSchemasForTopicEnv(), replaced by loader.getSchemasForTopicRegistry()

Any additional details or context that should be provided?

• Closes #574

Pull request checklist

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

Tests

• [ ] Added new
• [x] 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