feat: Hide email sign up button if specified

[adangil97]

[Hey there! So you want to contribute to FirebaseUI? Before you file this pull request, follow these steps:

• Read the contribution guidelines.
• Run ./gradlew check to ensure the Travis build passes.
• If this has been discussed in an issue, make sure to mention the issue number here. If not, go file an issue about this to make sure this is a desirable change.
• If this is a new feature please co-ordinate with someone on FirebaseUI-iOS to make sure that we can implement this on both platforms and maintain feature parity.
  ](feat: Hide email sign up button if specified)

[demolaf]

Hello,

Thank you for taking the time to submit this feature request.

We are happy to let you know that a resolution is already planned and on the roadmap for our upcoming new version.

We are actively working on the new SDK and will be sure to update this feature request once we have a public beta or a release candidate that includes this functionality. Please watch the main repository for the latest release.

In the meantime, we will keep this feature request open for tracking.