Allow override support for auth endpoints for proxy abilities

[jostster]

This PR adds the ability to override the identitytoolkit auth api and securetoken auth api so that organizations can place a proxy behind a firewall for control against attackers. #11858

[mTcirulnikov]

thanks

[paulb777]

Thanks @jostster for the PR and issue description!

@renkelvin Please evaluate if we want to move forward and want to make an API proposal.

[jostster]

Thanks @jostster for the PR and issue description!

@renkelvin Please evaluate if we want to move forward and want to make an API proposal.

Thanks @paulb777 . Could you clarify what would be needed for the API Request? There shouldn't need to be any backend api changes as this just routes the client to a proxy behind a firewall that then routes it to the regular firebase api endpoints after appending a firebase key that has access to the identity tool kit and secure token api.

[paulb777]

@jostster Every public Firebase API needs to go through an internal approval process. We need to sort that out internally and we have everything we need from you. Thanks!

[jostster]

@renkelvin if you need any specifics on the need for this, please let me know and I can provide some support tickets with Google that led us to this decision.