pw_rpc: Use pw_function callbacks in RPC client

This updates the nanopb RPC client to use pw::Function for callbacks
instead of custom virtual interfaces.

Change-Id: I7c548cde00223ec32e45cc831fe7f9f79577842b
Requires: pigweed-internal:13201
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48484
Reviewed-by: Wyatt Hepler <hepler@google.com>