[FR] Expose MFA info in UserRecord

**Is your feature request related to a problem? Please describe.**
[UserRecord.java](https://github.com/firebase/firebase-admin-java/blob/master/src/main/java/com/google/firebase/auth/UserRecord.java) does not parse a user's multi factor enrollment information, which is documented [here](https://cloud.google.com/identity-platform/docs/admin/manage-mfa-users).

```javascript
{
uid: 'some-uid',
// ...
multiFactor: {
enrolledFactors: [
// 2FA with SMS as 2nd factor.
{
uid: '53HG4HG45HG8G04GJ40J4G3J',
phoneNumber: '+16505551234',
displayName: 'Work phone',
enrollmentTime: 'Fri, 22 Sep 2017 01:49:58 GMT',
factorId: 'phone',
},
],
},
};
```
**Describe the solution you'd like**
Add a `mfaInfo` field to `UserRecord`. Example from the Node.js SDK: https://github.com/firebase/firebase-admin-node/blob/master/src/auth/user-record.ts#L78