Implement new dehydration format for Dehydrated Devices

[BillCarsonFr]

Currently, the dehydrated device is just a libolm-style pickle, and contains several unnecessary fields (some of which wouldn't make sense for other implementations). The MSC already has comments on these fields.

### Tasks
- [ ] #2602
- [ ] https://github.com/matrix-org/vodozemac/issues/196
- [ ] https://github.com/matrix-org/matrix-rust-sdk/issues/4185

[richvdh]

It's somewhat unclear what is in scope for this issue (as opposed to the wider issue, #922). @uhoreg: could you clarify the description?

[uhoreg]

This issue is only about changing the format used for storing the dehydrated device, whereas 922 tracks all the dehydration work.

[andybalaam]

All sub-issues are done, so marking as done.