[Versioning] Versioning specification

[laurabelinfante]

Summary

Define how API servers signal which version(s) of the CommonGrants protocol they support and how clients can optionally request a specific version. Covers version headers, discovery, negotiation, and error handling for unsupported versions.

Acceptance criteria

• A versioning specification defines how servers advertise supported versions and how clients request specific versions
• The specification covers version headers, discovery, negotiation, and error handling