Adapt to OCM spec for remote shares

[enriquepablo]

See #57152 #57166

Summary

This is a preliminary PR for discussion. If this goes through there are some things missing with which we'll be grateful to get some guidance:

• docs and tests
• keeping retrieved access tokens in the db
• removing tokens when removing shares

There is also the question that for access tokens, we are keeping a reference to the corresponding refresh token in the uid field of the access token db record.

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[enriquepablo]

Disclaimer: Some of the code in this PR (mainly regarding the tests) was generated by Claude Opus 4.5. I have provided it with very specific prompts, and thoroughly reviewed the output.