pw_protobuf: Encoder rework

Rewrites the pw_protobuf encoder to be oriented around pw_stream
compatibility.

Improvements:
 * Only the deepest nested encoder can be used at any given moment.
   Using a parent encoder triggers a crash rather than silently
   corrupting data.
 * Nested depth is bounded by scratch buffer size and stack usage rather
   than template arguments.
 * Can write to pw::stream::Writer objects without any temporary buffer.
 * Encoders can be explicitly finalized to allow returning to the parent
   encoder for further modifications without needing to create scopes to
   handle RAII behavior.
 * Proto encode interfaces can standardize on the StreamingEncoder as
   any StreamingEncoder can nest until the temporary buffer is
   exhausted.
 * Better handling of endianness.

Change-Id: I3c50abbf2fe28328a9cb1aa527cf829abe583218
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/40245
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Keir Mierle <keir@google.com>