pw_rpc: Rename base_server_writer files

This file rename was done in a separate change from the class rename so
Git would detect these as file moves rather than new files.

Change-Id: I37b6168b142197628b6694736ce0fbe36e44b263
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/49201
Commit-Queue: Wyatt Hepler <hepler@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>