Allow custom ClientInfo

[busunkim96]

Clients are currently always using the default ClientInfo.

gapic-generator-python/gapic/ads-templates/%namespace/%name/%version/%sub/services/%service/transports/base.py.j2

Line 95 in 1d08e60

client_info=_client_info,

Allow ClientInfo to be passed as a kwarg to the constructor and use it when provided.

googleapis/python-kms#37

[software-dov]

Fixed by #573