Adds a PersistentBuffer object that acts as a persistent form of a
MemoryWriterBuffer.
Change-Id: I61db1a77476ceececd088fd60bc10bfaf1b99e0e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/39360
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Armando Montanez <amontanez@google.com>
