rpk: provide a way to print a schema from the registry

[r-vasquez]

Who is this for and what problem do they have today?

We introduced rpk registry schema get in 23.3, but there is no way to print the schema, just the metadata.

What are the success criteria?

Provide either a flag --print/--print-schema or a new command. rpk registry schema get returns an array, so we should handle the case for queries that return multiple schemas. Probably printing all schemas is fine.

Why is solving this problem impactful?

Improve the UX.

Additional notes