Use separate response messages in our gRPC API

[emilk]

It is best practice to use a different response message to each rpc call, as that allows us to add more fields to the response in the future without sacrificing compatibility.

That is, instead of

    rpc QueryCatalog(QueryCatalogRequest) returns (stream DataframePart) {}
    rpc RegisterRecording(RegisterRecordingRequest) returns (DataframePart) {}

we should have

    rpc QueryCatalog(QueryCatalogRequest) returns (stream QueryCatalogResponse) {}
    rpc RegisterRecording(RegisterRecordingRequest) returns (RegisterRecordingResponse) {}

[grtlr]

I can do this as part of #8566.

@zehiko @jleibs Right now, the gRPC API uses a lot of dataframes instead of dedicated types. I think we should start to define more dedicated types (and optionally add a request parameter to return a data frame if still required).

[zehiko]

the most recent API changes were made as part of https://github.com/rerun-io/dataplatform/issues/74 (which is good to read for understanding the motivation behind the changes).