Write proposal to restrict contact information field to email address (and make it mandatory)

We should write a proposal to make sure ContactInfo is really just an email address. We could add a second step where we want to enforce it actually. Tor's man page says:

       ContactInfo email_address
           Administrative contact information for this relay or bridge. This line
           can be used to contact you if your relay or bridge is misconfigured or
           something else goes wrong. Note that we archive and publish all
           descriptors containing these lines and that Google indexes them, so
           spammers might also collect them. You may want to obscure the fact that
           it’s an email address and/or generate a new address for this purpose.

           ContactInfo must be set to a working address if you run more than one
           relay or bridge. (Really, everybody running a relay or bridge should
           set it.)

which is already pretty close to what I plan to propose. This is related to tpo/core/arti#870.

@gus is not done with the meta proposal yet, so I wait for that. Additionally, I am not sure yet how to split this up properly for some potentially needed tor-spec proposal.