feat(api-core): add client_cert_source to ClientOptions (#17)

* feat(api-core): add client_cert_source to ClientOptions

* Update google/api_core/client_options.py

Co-Authored-By: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>

* bump google-auth version

* update noxfile.py to fix docs problem

Co-authored-by: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
diff --git a/setup.py b/setup.py
index 820d5f4..30f83a6 100644
--- a/setup.py
+++ b/setup.py
@@ -31,7 +31,7 @@
 dependencies = [
     "googleapis-common-protos >= 1.6.0, < 2.0dev",
     "protobuf >= 3.4.0",
-    "google-auth >= 0.4.0, < 2.0dev",
+    "google-auth >= 1.14.0, < 2.0dev",
     "requests >= 2.18.0, < 3.0.0dev",
     "setuptools >= 34.0.0",
     "six >= 1.10.0",