Device Dehydration | js-sdk: store/load dehydration key

[BillCarsonFr]

Fixes #4483

Added basic support to cache created keys and read from store instead of in memory.
Then adding some basic signalling to DeviceDehydrationManager, helpful for tests and also for apps to have some feedback on device rehydration (reports progress for example)

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).

[uhoreg]

I only did a quick review because the the compiler errors are distracting

[andybalaam]

Looks reasonable to me, but definitely needs review from people who are closer to the dehydrated devices design.

[uhoreg]

I think that I've addressed all of @richvdh 's comments.

(Of course, this can't be merged until #4634 is fixed)

[richvdh]

suggest an early return here, to avoid two calls to getCachedKey and dodgy ! operators

Suggested change

	const cachedKey = await this.getCachedKey();
	if (!cachedKey) {
	const cachedKey = await this.getCachedKey();
	if (cachedKey) return cachedKey;

(can we then inline getCachedKey?)

[uhoreg]

SonarCloud says I need to add more tests, but should be ready for re-review otherwise

[richvdh]

A few nits, but LGTM.

And yes, looks like we need a test for loading the key from 4S?