Standard: Disable Signin for Shared Mailboxes

[dbeta]

Is your feature request related to a problem? Please describe.
A useful standard we already keep for our tenants is disabling of shared mailboxes. I'm unsure why Microsoft doesn't do this already, but my understanding is that shared mailboxes can be logged into. So it is best practice to ensure they are disabled.

Describe the solution you'd like
Create a standard to make sure all shared mailboxes have sign-in disabled.