Feature-Gate Server and Client derives

[insipx]

Is your feature request related to a problem?

We don't have server and client behind feature gates, which means even if a client only wants client code, they will pull in all server code.

Describe the solution to the problem

We should feature-gate the client and server code similar to how it is in lib_didethresolver: https://github.com/xmtp/didethresolver/blob/main/lib/src/rpc.rs

Note: we should make sure that the client feature gate in xps also only compiles lib_didethresolver with the client feature or no features (as to not pull in any rpc code from lib-didethresolver)

Describe the uses cases for the feature

Libxmtp compiles to wasm, and its important to remove unnecessary code from the resulting blob.