Add gRPC API Gateway config schemas

[peymanmortazavi]

Adding schema for a protobuf plugin I am about to release soon.

Github repository: https://github.com/meshapi/grpc-api-gateway

I read through the contribution notes and I think I checkmarked everything but if I have missed some note, I'll be happy to make the necessary changes. Perhaps a pull request description is not a great place to indicate this but I'm very impressed by this project, it's really helpful, the guidelines are concise and descriptive. I hope I and hopefully future contributors to the project can capture the same quality.

NOTE: I know draft-07 is recommended but the tool I used to generate the schema supports draft-04 only. I might at some point validate that it can be upgraded to draft-07 and update the version.

[github-actions[bot]]

Thanks for the PR!

This section of the codebase is owned by @madskristensen and @hyperupcall - if they write a comment saying "LGTM" then it will be merged.

[madskristensen]

Thanks