Increase 'upload schema' feature visibility through 'pick a file' sort of flow given a single schema registry.

[jlrobins]

1. Add a new '+' icon action in the schemas view titlebar. Will start the flow from the perspective of this schema registry, but unknown file.
2. Starts a 'Pick a file ...' sort of flow like used for picking PEM files for connection TLS certs, https://github.com/confluentinc/vscode/blob/6b14b4a853a6c50594f267ad3ec84701a1910f29/src/commands/connections.ts#L35 , but filtering for avsc|json|proto files.
3. Ends up calling into existing functionality for uploading said schema contents into the given schema registry.

[derek1ee]

I recommend offering a few different options after clicking the "+" button:

• Choose a file
• New JSON schema
• New AVRO schema
• New protobuf schema

Choosing "New * schema" will open an editor with blank or preferrably a basic placeholder content (similar to Confluent Cloud).

This capability should be available on both the schema registry level (create a brand new schema w/ file) as well as on the schema level (create a new version with file, in which case the schema type selection should be skipped since it's already determined).

Since we should schemas as child tree items under the topics view, the same capability of creating a new version with file should be made available there as well.