Implement remote storage sync

Follow-up to https://github.com/JabRef/jabref/pull/9748. While https://github.com/JabRef/jabref/pull/9748 is for a plain http server, this one implements the JabRef sync to an online service. Details on sync at https://github.com/koppor/jabref/blob/sync-mwe/docs/code-howtos/remote-storage-jabdrive.md.

This is WIP. Missing at least

[ ] new data structure at update test
[ ] Add checksum from server in code
[ ] Add checksum from server to docs (meaning detailing https://github.com/koppor/jabref/blob/sync-mwe/docs/code-howtos/remote-storage-jabdrive.md)
[ ] Implement sync ^^

### Compulsory checks
- [ ] Change in `CHANGELOG.md` described in a way that is understandable for the average user (if applicable)
- [ ] Tests created for changes (if applicable)
- [ ] Manually tested changed features in running JabRef (always required)
- [ ] Screenshots added in PR description (for UI changes)
- [ ] [Checked developer's documentation](https://devdocs.jabref.org/): Is the information available and up to date? If not, I outlined it in this pull request.
- [ ] [Checked documentation](https://docs.jabref.org/): Is the information available and up to date? If not, I created an issue at <https://github.com/JabRef/user-documentation/issues> or, even better, I submitted a pull request to the documentation repository.

koppor / jabref

Implement remote storage sync #642