[depot_tools][owners] Add owners client to depot_tools.
[depot_tools][owners] Add owners client to depot_tools.

Add an owners API that will be used by Depot Tools to
interact with OWNERS files.

The API will be implemented using both owners.py and the
Gerrit Code-Owners plugin REST API. All Depot Tools code
will be modified to use this API.

Change-Id: I7cf059a0895dbae105a2f0b26568fd7b47068f43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2517985
Reviewed-by: Anthony Polito <05bb588aa9020353543fee7ac4e6e0a3d0d6f6ed@google.com>
Commit-Queue: Anthony Polito <05bb588aa9020353543fee7ac4e6e0a3d0d6f6ed@google.com>

CoherentLabs/depot_tools,CoherentLabs/depot_tools