grpc-gateway powered REST API for MultiCluster Allocation

[markmandel]

Is your feature request related to a problem? Please describe.

For users for which gRPC isn't a viable option, it would be handy to have a REST based API as well for the gPRC API.

Describe the solution you'd like

We use grpc-gateway to generate the REST API, but also use the same certificates to authenticate the endpoint as we would for the gRPC endpoint.

We can also generate a swagger.json so that people can generate REST clients as well.

Describe alternatives you've considered

Having no REST endpoint. But we're seeing bugs around SSL cert management in other languages, and this would provide a workaround for any other issues we have in the future.

Additional context

I would recommend completing #1331 first.

It might also be worth updating grpc-gateway at some point as well, we're quite a few releases behind.

[Davidnovarro]

REST API should be ok.

[robbieheywood]

+1 - we'd really like this!