Backends for the pw_rpc/system_server facade were defined in the pw_rpc
module, preventing targets from customizing their server configurations
and channels. This change moves the backends into the respective target
directories. Additionally, pw_unit_test's RPC main is updated to use the
system server.
Change-Id: I670e13dd178ab8a7fc8dfa8fade3f82a46d5208c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/28500
Reviewed-by: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Alexei Frolov <frolv@google.com>
