Schema types should be informed from SR API

[rohitsanj]

List of schema types for an SR cluster must be informed from the GET /schemas/types response, but still limited to only those that we currently support (i.e, AVRO, PROTOBUF and JSON).

Here's where we "hardcode" the list of schema types -

vscode/src/quickpicks/schemas.ts

Line 105 in 87b6906

const schemaTypes = Object.values(SchemaType);