role_based_login sample

[shamylkashif]

This PR adds a new Flutter sample named role_based_login under the experimental folder.
The sample demonstrates role-based login using Firebase Authentication with separate dashboards for admin and users.

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[shamylkashif]

CLA signed

[ericwindmill]

Thank you for the contribution @shamylkashif. Unfortunately, we are no longer adding large samples or samples that rely on libraries per the Contributing guidelines.