Customize SAML Authnrequest generated by firebase

### [REQUIRED] Describe your environment
Operating System version: Windows 10
Browser version: Chrome 87.0.4280.88
Firebase SDK version: 8.0.0
Firebase Product: auth

### [REQUIRED] Describe the problem
I need to signin users with an external identity provider through SAML 2.0 protocol.
I tried to use the firebase auth signInWithPopup with a custom SAML auth provider for signing in users.
In particular, I set up the new provider in the google console giving the certificate and all the required data.

The problem is that firebase automatically generates the AuthnRequest which is given to the Identity provider.
Since the identity provider I need to use is based on specifications which require the AuthnRequest to have specific information, we cannot use the firebase auth api. In fact we would need in the AuthnRequest:

- NameQualifier and Format attributes in the Issuer
- NameIDPolicy
- RequestedAuthnContext

The missing information make the identity provider to reject the request unfortunately.

It would be amazing to be able to customize the AuthnRequest while signing in users with SAML.
Would it be possible to do that? It would be really helpful...

Thank you,
Valentino