Device Dehydration | js-sdk: provide functions to allow application to create dehydrated devices on restart

[BillCarsonFr]

Follow up of #4483
part of element-hq/element-meta#2603

When the SDK starts, use the function from matrix-org/matrix-rust-sdk#4186 to check if there is a dehydration key cached, and if so, schedule periodic device dehydration. May want to wait until a sync is done, so that we can see if the dehydration key got reset, so we don't create a dehydrated device using an old key.

When the application starts, it should check if there is a dehydration key cached, and if so, schedule periodic device dehydration. The SDK needs to provide functionality so that the application can do so.