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>
4 files changed