Research: use micro gateway instead of self generated web handler

[kulmann]

We want to look into
a) using the API Gateway for proper endpoints, instead of using protoc-gen-microweb for web endpoint code generation https://micro.mu/docs/api.html#rpc-resolver
b) being able to run the service standalone, without the gateway. With proper protobuf endpoint annotations, we might be able to use the generated api.Endpoints without the gateway.