feat(rst): add support for user extended attributes

[swartzn]

This change requires the protobuf changes from ThinkParQ/protobuf#63

What does this PR do / why do we need it?

Required for all PRs.

The ability to synchronize user extended attributes along with the file contents.

Related Issue(s)

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Are there any specific topics we should discuss before merging?

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What are the next steps after this PR?

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Checklist before merging:

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