pw_rpc: Add packet processors to pwpb testing context

This CL adds an ability to customize packet processing in
pwpb ClientServerTestingContext (and its threaded variant).

This can be useful when the project has its own packet processing (e.g.
one that offloads all rpc handling to a custom thread pool) and wants to
simulate this behavior in tests as well.

Change-Id: I7c4347e5946e28bec11674918f2dfc175fa6a42a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/117091
Pigweed-Auto-Submit: Dennis Kormalev <denk@google.com>
Reviewed-by: Taylor Cramer <cramertj@google.com>
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