Add a public interface for PlusAddressService

`PlusAddressService` is an abstract class that inherits from `AutofillPlusAddressDelegate` and implemented by `PlusAddressServiceImpl`.

`FakePlusAddressService` implements `PlusAddressService` for the tests.

Often times, `AutofillPlusAddressDelegate` is updated and `FakePlusAddressService` does not get to know about it, leading to failures in iOS egTests since, `FakePlusAddress` is used instead of `PlusAddressService` in iOS. This change would prevent such failures in the future.

Bug: 370495073
Change-Id: If976977056f62bf2e43c5d2dcc6102b5acd66593
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5899135
Reviewed-by: Jan Keitel <jkeitel@google.com>
Reviewed-by: Norge Vizcay <vizcay@google.com>
Commit-Queue: Vidhan Jain <vidhanj@google.com>
Cr-Commit-Position: refs/heads/main@{#1363110}