Original CalDAV driver was written almost a year ago and code quality,
patterns were not up to the mark including having no test coverage, this
PR does the following:
- Splits the monolithic driver into isolated utilities with test
coverage
- Adds support for syncing legacy servers by checking if server supports
`syncCollection` and branches into two sync methods
`fetchEventsViaSyncCollection` or `fetchEventsViaCtagEtag` with this I
believe our driver is feature complete
Real testing report
| Provider | Server | Sync method | Auth |
| --------- | ----------------- | -------------------- | ------ |
| iCloud | Apple's CalDAV | sync-collection | Basic |
| Nextcloud | sabre/dav | sync-collection | Basic |
| all-inkl | sabre/dav (older) | ctag + etag fallback | Digest |
