Feature: CTA to 'create value schema' on a schema-less topic

CTA bound to a schema-less topic to invite the user to start creating a schema. Ask for the schema type, then open up a new document of that language type / extension. Store metadata about the buffer relating it to the cluster + topic, so that then when time comes to upload the schema, we know where to send it to and what to default the name to be (`$topic-value`).

Having ported the schema upload CTAs to be codelens actions would be nice, 'cause then with metadata already assigned, would just need to interact with a single codelens for 'upload' --- the SR and subject name codelenses would be preset.

Would make bootstrapping a new topic + schema pair simpler.