protoc-gen-grafbase-subgraph: proper @derive support

[tomhoule]

You can now create federation-style entity references using the composite_schemas_entity option on messages.

• Automatically generates reference fields with @derive and @is directives
• Creates stub entity types with @key directives if the target entity isn't already being rendered.
• Supports custom relation field names with the field parameter
• Enables entity references from a single id or multiple fields in federated schemas, including list of ids.

closes GB-9489

[Finistere]

Added comments in slack

[linear]

GB-9489 protoc-gen-grafbase-subgraph: @derive support

[tomhoule]

Addressed everything we discussed @Finistere , merging now.