[Feature] Support SRV records for the PDS and for handle resolution

[UpcraftLP]

This is in response to a discussion on the API Touchers discord.

Currently, it is possible to run a PDS on nonstandard ports by passing the port in the URL (http(s)://mydomain.tld:1234/), but handle resolution via /.well-known/ is an entirely unrelated mechanism and assumes standard ports for http(s).

This could be resolved by making use of DNS SRV records, which map a named service to a port.

Something like _atproto._tls.domain.tld maybe?

(note: the protocol part cannot be freely chosen on most DNS providers, ex. cloudflare only allows _tcp, _udp and _tls)

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If the PDS also respects this, this would additionally allow for running a PDS on a domain that already has an (unrelated) webserver listening on ports 80/443.

[bnewbold]

A couple initial thoughts:

• would really like to keep the number of handle resolution options and mechanisms as small as possible. having two methods is already additional complexity and somewhat painful
• if DNS is already involved, why not just do DNS-based handle resolution with the TXT record? the well-known endpoint is basically only supported to help folks running many-user platforms who don't have DNS automation set up
• I think that having a mechanism to handle /.well-known/ endpoints for additional or unrelated purposes is a pretty established functionality for web hosting. so even if some other web service (eg, wordpress) is already running on a domain, there is often a mechanism to handle well-known endpoints (eg, for Let's Encrypt certificate validation, and many other standard use-cases)