Add configuration support for Fedify's split-domain WebFinger capability

[jfietkau]

Fedify supports specifying a handle domain that's separate from the host domain as of version 1.5.0.

It would be nice to surface this functionality in Hollo as a configurable setting.

[jfietkau]

Possible design concerns:

• Should this configuration setting happen at install time and then become immutable? Mastodon's and GoToSocial's documentation suggest that other ActivityPub implementations may assume unchanging handle domains, and that changing an existing Actor's WebFinger handle after it has already started federating may cause interoperability issues. I'm not certain if this is accurate.
• GoToSocial restricts the handle domain to have to be on the same root domain as the host, with the idea being that removing subdomains is the main use case. I think this is an unnecessary restriction and that server operators should be able to pick any handle domain they control, but YMMV.

My WebFinger Canary has more documentation on this feature.