Add emails whitelist for "external" users

[rsc1975]

We are evaluating Rocket.chat for our organization and It's cool, probably we use it, however I miss a key feature for enterprise use, the possibility to add external users to our organization, when we have restricted the registry to our company domain:

The proposal is:

• Add an additional "rule" in the Registry section to allow specific emails, something like "emails whitelist", so an admin can include guest users outside the company domain.

I've comment out an issue that is related with this, so I know that there is a workaround, but It's a bit uncomfortable: https://github.com/RocketChat/Rocket.Chat/issues/9139