pw_log_zephyr: Add support for other backends

- Add config to enable overriding the Zephyr Log Macros to PW_LOG*
  macros.
-- Make a library that includes these overriding headers and use it
   in libraries that depend on this
- Add a new PIGWEED_LOG choice to enable this redirection for users
  that provide their own pw_log backend.
- Add a new PIGWEED_LOG config to use the pw_log_string backend for
  users that provide their own pw_log_string.handler backend.

Change-Id: Ibeee67b1ac572cbb0c09afa78ec47b5fd1d7eb96
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/304673
Pigweed-Auto-Submit: Carlos Chinchilla <cachinchilla@google.com>
Docs-Not-Needed: Carlos Chinchilla <cachinchilla@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Carlos Chinchilla <cachinchilla@google.com>
Reviewed-by: Kesavan Yogeswaran <hikes@google.com>
Reviewed-by: Al Semjonovs <asemjonovs@google.com>