FR: Add MANDATORY state in MFA config

[Ziqi Xiao]

[REQUIRED] Step 2: Describe your environment

• Operating System version: _____
• Firebase SDK version: _____
• Library version: 4.13.0_
• Firebase Product: auth_ (auth, database, storage, etc)

[REQUIRED] Step 3: Describe the problem

For the Multi-factor config state only Enable and Disable is supported, can we also add MANDATORY here shown from this link.