Backup Preference Option for providers who support both backed-up and non-backed up credential

[akshayku]

Closes #2252

This PR provides a way for an RP to indicate their backup preference regarding the credential being created at registration time.

Note: Given the nature of different options provided by the providers/authenticators, their capabilities, user choices etc., RP must expect both backed-up and non-backed-up credentials in the registration responses.

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