Require verified certificates for federation (MSC1711)

[anoadragon453]

MSC1711 states that federation connections must only be made to homeservers that have a valid X.509 (SSL) certificate. The specification states this as well, thus Dendrite must enforce it.

[JorikSchellekens]

Will we support this: "Servers can additionally skip the certificate validation for a given whitelist of domains or netmasks for the purposes of testing or in networks where verification is done elsewhere, such as with .onion addresses"