Support adding schema to a schemaless topic

[derek1ee]

If a topic has no schema, then we should provide an option (e.g. in context menu or as a child tree item) to add a schema. Selecting it would first prompt users to select type (json/avro/proto) and then present user with a placeholder file similar to the Confluent Cloud experience.

When choosing to upload, the said schemaless topic should be pre-selected to make upload easier.