Introduce affiliations adapter for plus addresses.

The CL  introduces the initial functionality an affiliations source
adapter for plus addresses. Observing new plus address events (e.g. add,
remove) will be added once sync capabilities are integrated.

Bug: b/324553908
Change-Id: Id1337a89f06ecd6b8642ab09db691e84bcc5a3d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5361517


Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Norge Vizcay <vizcay@google.com>
Reviewed-by: Viktor Semeniuk <vsemeniuk@google.com>
Cr-Commit-Position: refs/heads/main@{#1271474}