Add per store key assertion back

[unional]

Create store with ID and key assertion

Get store with ID and key.

The library can randomly generate a key and use the key assertion to make sure the access is legit.

Within MFE, the host can provide a key assertion and key generation function to the mfe.

[unional]

One question remains: should it be async to accommodate server side validation?

My gut feeling is no. That will be too heavy.
But the question is what is the actual use case and is there a way to achieve high security without the async capability.