Authentication: SAML Custom Attributes are not supported

It would be nice if the auth emulator supported a toggle of where its custom claims are applied. In the case of the Identity Platform's SAML provider, it needs to populate in `firebase.sign_in_attributes`, such as:

```json
{
"iss": "https://securetoken.google.com/myapp",
"aud": "myapp",
"auth_time": 1628461622,
"user_id": "JaWznJClYW5nvHnePGRB6RwVh1",
"sub": "JaWznJ6QW5nvHnePGRB6RwVh1",
"iat": 1628461622,
"exp": 1628465222,
"email": "john@myapp.com",
"email_verified": true,
"firebase": {
"identities": {
"saml.myapp": [
"john@myapp.com"
],
"email": [
"john@myapp.com"
]
},
"sign_in_provider": "saml.myapp",
"sign_in_attributes": {
"uid": "12346",
"mail": "john@myapp.com",
"surname": "Gasper",
"givenName": "John"
}
}
}

A work around would be to allow me to override the claims key check forbidding the use of the `firebase` key and merge my block with the default:
```json
{
"firebase": {
"sign_in_attributes": {
"uid": "12346",
"mail": "john@myapp.com",
"surname": "Gasper",
"givenName": "John"
}
}
}
```

I suspect this might be helpful in working with the other Identity Platform auth providers too.