Add user_agent property to ClientInfo

[tswast]

This provides a way for partners to define a prefix identifying their
tool or application, as required by many cloud partnership agreements.

[busunkim96]

A pytype check is failing. (I don't think it's related to this change?)

[3/26] check google.api_core.datetime_helpers
FAILED: /tmpfs/src/github/google-cloud-python/api_core/pytype_output/pyi/google/api_core/datetime_helpers.pyi
pytype-single --imports_info /tmpfs/src/github/google-cloud-python/api_core/pytype_output/imports/google.api_core.datetime_helpers.imports --module-name google.api_core.datetime_helpers -V 3.6 -o /tmpfs/src/github/google-cloud-python/api_core/pytype_output/pyi/google/api_core/datetime_helpers.pyi --analyze-annotated --nofail --quick /tmpfs/src/github/google-cloud-python/api_core/google/api_core/datetime_helpers.py
File "/tmpfs/src/github/google-cloud-python/api_core/google/api_core/datetime_helpers.py", line 23, in <module>: Couldn't import pyi for 'google.protobuf.timestamp_pb2' [pyi-error]
  Can't find pyi for 'google.protobuf.descriptor_pb2.FieldOptions', referenced from 'google.protobuf.descriptor_pb2'
For more details, see https://google.github.io/pytype/errors.html#pyi-error.

@rchen152 Do you have any insight here?

[rchen152]

It looks like you've been bitten by google/pytype#150 - pytype has trouble loading some third party pyi files, including the google.protobuf ones, it seems.

The easiest workaround is to add disable = pyi-error to the pytype section of setup.cfg.