Whitelist for federation

[MakaryGo]

Is your feature request related to a problem? Please describe.
It would really make the decission of enabling federation on the instance easier if it would be possible to hand pick the servers that I want to federate with.

Describe the solution you'd like

1. I would love to be able to just put together a list of instances that I want to federate with, even if it's just a text file on server
   1.1. It would be perfect it it could be available as a setting from the control panel at some point

Describe alternatives you've considered
It would be possible to just blacklist servers as needed, I suppose, but this requires way more energy and labour put into the moderation, and usually it's done on a volunteer basis.

Additional context
It was discussed on Matrix, and @BentiGorlich not only agreed it's a cool idea, but also said that it would be relatively easy to implement.

[melroy89]

Some whitelist next to the banned list in the GUI here would be the most logical place to add it to the UI: /admin/federation

Add adding some informational text above it saying something like: "When using a whitelist, the banned list is ignored" after all, when creating a whitelist you do not need a ban list anymore, you have a whitelist after all.

[vaguerant]

Re: Indicators for the ban list, you could instead or additionally grey out the currently unused list, to give a visual indication that it is not being used. So whitelist is greyed out when ban list is enabled and vice versa.