python: Consolidate schema types

[gasmith]

This change didn't go as far as I thought it would when I wrote the ticket.

It turns out that the (message_encoding + Schema) we use for channels is really a (message_encoding + Optional[Schema]) because channels can be schema-less. The same is not true for service request/response schemas, which are mandatory.

Changes here:

• Dropped SchemaDefinition.
• The Channel constructor takes a message encoding and a Schema.
  • The message encoding is optional (and ignored) if the schema is a JSON schema.
  • The schema must be specified as a keyword argument, but may be None for schemaless.
• Plumbed Schema through the BaseChannel constructor.
• Removed the rejection of schemas with empty strings as encoding/data. If we really want to be that pedantic, we should do it in rust too.

I'm not thrilled with the result, but I thought I'd throw it up for review anyway. Feel free to nack.

[linear]

FG-10612 python: Consider consolidating schema types

[bryfox]

I think this is an improvement. I started to write more feedback, but your added alternative seems to address this.

Removed the rejection of schemas with empty strings as encoding/data.

👍 ...I think this was a holdover from an early python version; we should be consistent, and mcap allows empty schemas.