refactor: improve the TonicRpcClient so it doesn't rely so much on feature flags

[tomyrd]

When deduplicating the code for the rpc client implementations in #744 we had to add a lot of conditional compilation for it to work, we should look for a way where we don't duplicate code but also don't rely so much on feature flags to solve compilation problems.

I still don't love the fact that we need to rely on feature flags this much in order to avoid falling into problems such as duplicate declarations, missing imports/exports, etc. However I do think that the deduplication is a net win here.
I briefly tried to generalize it without the feature flags but it becomes verbose too fast (hard to make generic over ApiClient because of the inner guards defined by tonic). So could we create an issue to keep this in the backlog as we merge this?

Originally posted by @igamigo in #744 (review)

Additional context and ideas can be found in this other comment.