fix: Make ChangeStreamRecord interface serializable

[jackdingilian]

Make ChangeStreamRecord interface Serializable so that beam can create Coders for it by default. ChangeStreamMutation, CloseStream, and Heartbeat all already implement Serializable.

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Fixes #<issue_number_goes_here> ☕️

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[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

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