Device Dehydration: Cache the dehydration key

[BillCarsonFr]

Part of element-hq/element-meta#2603

Currently, the dehydration key is only kept in memory, which means that when the client gets restarted, it will no longer have the dehydration key, and will not be able to create new dehydrated devices. We need to cache it in the crypto store so that the SDK can fetch it on statup.

• Add functions to store/get dehydration key: store the key, fetch the key (or None if no key is stored), delete the key
• sqlite migration
• indexeddb migration