Add `google/gnostic` as a managed module

[unmultimedio]

Fixes #482

[unmultimedio]

Tested: #550

[unmultimedio]

Looking at this thread, we are turning this PR into a draft while that protos restructuring happen, so we don't deviate from the original intended paths/pkgs names from the author.

For now, in the public BSR, interested users can depend on the module that the author is already maintaining here: https://buf.build/gnostic/gnostic/docs/main:gnostic.openapi.v3, and for Pro and Enterprise customers we can recommend buf export and buf push in the meantime.

[unmultimedio]

Closing it for now. We can reopen/reevaluate later.