Add Channel API code owners for future reviews (#5594)
Add Channel API code owners for future reviews (#5594)

Refers #5583 accordingly [this](https://github.com/GoogleCloudPlatform/python-docs-samples/pull/5593#pullrequestreview-623726650) comment.

Co-authored-by: gcf-merge-on-green[bot] <6c7a4de7cdf40496888e303ea3f489f3cc742362@users.noreply.github.com>
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