feat: add ability to provide own mock id

[malakagl]

A solution that will fix. #295

Allows users to define their mock IDs in stubs definitions with mockId.

[gwleclerc]

Thanks for the PR @malakagl! 🙏

We're going to decline this one: client-provided IDs go against a core design choice — PRINCIPLES.md §3.6, IDs are opaque, randomly generated strings; nothing should depend on their shape or generator.

For the actual need in #295: GET /mocks already returns each mock's id under state.id, so you can read the generated IDs after registering your mocks — no need to set them yourself.

Thanks again for contributing!