[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/+/2517985Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Anthony Polito <apolito@google.com>

owners_client.py

+# Copyright (c) 2020 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+
+class OwnersClient(object):
+  """Interact with OWNERS files in a repository.
+
+  This class allows you to interact with OWNERS files in a repository both the
+  Gerrit Code-Owners plugin REST API, and the owners database implemented by
+  Depot Tools in owners.py:
+
+   - List all the owners for a change.
+   - Check if a change has been approved.
+   - Check if the OWNERS configuration in a change is valid.
+
+  All code should use this class to interact with OWNERS files instead of the
+  owners database in owners.py
+  """
+  def __init__(self, host):
+    self._host = host
+
+  def ListOwnersForFile(self, project, branch, path):
+    """List all owners for a file."""
+    raise Exception('Not implemented')
+
+  def IsChangeApproved(self, change_number):
+    """Check if the latest patch set for a change has been approved."""
+    raise Exception('Not implemented')
+
+  def IsOwnerConfigurationValid(self, change_number, patch):
+    """Check if the owners configuration in a change is valid."""
+    raise Exception('Not implemented')