Add plugin `Api-Key` for auth

Created by: goshik-e

https://github.com/perxteam/perxis/issues/2601

examples/auth_api_key.py

+import logging
+
+import grpc
+
+from perxis.auth import APIKeyPlugin
+from perxis.collections import collections_pb2_grpc, collections_pb2
+
+API_KEY = '0Xp7bYBqYmIUqwZcNRMtRCtDBaheFdAc'
+SPACE_ID = 'c2qcp9cuaccmpj8lmom0'
+ENV_ID = 'master'
+TARGET = 'envoy.perxis.pt.perx.ru:443'
+
+
+def main():
+    api_key_plugin = APIKeyPlugin(API_KEY)
+    call_credentials = grpc.metadata_call_credentials(api_key_plugin, name='api-key')
+    channel_credentials = grpc.ssl_channel_credentials()
+    composite_credentials = grpc.composite_channel_credentials(
+            channel_credentials, call_credentials
+    )
+    with grpc.secure_channel(TARGET, composite_credentials) as channel:
+        stub = collections_pb2_grpc.CollectionsStub(channel)
+        collections = stub.List(collections_pb2.ListRequest(space_id=SPACE_ID, env_id=ENV_ID))
+        print(collections)
+
+
+if __name__ == '__main__':
+    logging.basicConfig(level=logging.INFO)
+    main()