Add async API signatures

[abergs]

To support async / await callbacks.
Also add Async callback signatures.

Should just mirror existing names but append "Async", e.g. MakeAssertionAsync

[abergs]

Or should we leave the storage of counter / public key creds responsibility to the call site? Might be better when I think about it.

Async might be needed for metdata services however. Leaving this open.

[aseigler]

Wouldn't hurt to provide a sample storage facility for counter/public keys in the form of an in-memory database. Would be excellent for proof of concept.

[abergs]

Sure, but I'm still leaning to have it at the call site as separate methods. Async might be needed in the callback for ICredentialUniqueToUser though (since that will need to scan storage)

[abergs]

Done. DevStorage added in 2d866a5#diff-43a3af1c06c320b60ef91be4cfbf21f0

Unfortunalte ReadOnlySpans are not allowed when methods are async which increases byte[] copying.