Updates UserRecord to parse multi-factor related information from the GetAccountInfo server response.

First step in supporting multi-factor authentication with SMS as a second factor.

* Adds ability to import users with phone second factors.

* Addresses review comments.

* Adds multi-factor support for updateUser.
This will allow the ability to update the list of second factors on an existing user record.

* Extends createUser to support multi-factor user creation.

Note that only phoneNumber and displayName are allowed to be passed in this operation.

Adds relevant unit and integration tests.

This capability is only available in staging.

…gs. (#752)

* Adds backend multi-factor auth related errors and their client mappings.

…on. (#766)

* Defines the d.ts multi-factor related comments for reference generation.

* Adds missing multi-factor related claims to the `DecodedIdToken` interface.