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.

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1. MSC3814: Document new dehydrated device format (probably just a concatenation of the various bits of information needed, similar to existing pickle format) #2602
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2. Dehydrated Device: Serialize and deserialize new format matrix-org/vodozemac#196
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3. Support Device Dehydration new format matrix-org/matrix-rust-sdk#4185
   encryption +1
   

[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.