Google Ads API v10_0 release

Remove support for Google Ads API v7.

- Google Ads API v9_0 release

- Update gRPC transport logic to replace "gapic" user-agent with "gccl"
- Update examples to support v9 changes
- Add new Performance Max examples
  advanced_operations/add_performance_max_campaign.py and
  shopping_ads/add_performance_max_retail_campaign.py

- Pin protobuf to < 3.18.0

- Add examples add_bidding_data_exclusion, add_bidding_seasonality_adjustment
- Style updates for examples that use search_stream
- Fix hardcoded URLs for media in various examples

* New required `use_proto_plus` configuration parameter.

* Add support for Google Ads API v8_1.
* Remove support for Google Ads API v6.

- Bump the minimum Python version requirement to 3.7+

- Fix package name in base transport files
- Update examples to new "client.enums" syntax
- Move CSV reporting example to migration directory
- Various minor updates to examples and documentation

- Google Ads API v8_0 release.

- Deprecate Google Ads API v5_0.
- *NOTE* This will be the last release to support Python 3.6.

Only install dataclasses module when < Python 3.7

- Fix bug breaking Python 3.6 compatibility.

- Update unit test framework to Nox and add test coverage.
- Various updates to examples and GitHub issue templates.

Release Google Ads API v7_0

Deprecate Google Ads API v4_0
Add http proxy support

- Revamp of protobuf message interface to improve usability. See:

https://developers.google.com/google-ads/api/docs/client-libs/python/library-version-10
- Remove ResourceName utility
- Add ability to specify API version at client level, which overrides setting
  at service or type level.
- GoogleAdsClient.get_type is now an instance, not class, level method
- Add copy_from helper method to GoogleAdsClient.
- Add "enums" attribute to GoogleAdsClient for easier Enum accessing.
- Various updates to all examples