feat: discover from DNS SRV records

[caarlos0]

allow to find entrypoints from _ssh._tcp.DOMAIN SRV records.

refs #135

---

to try it out, create a DNS entry in a domain you own, for example _ssh._tcp.caarlos0.dev.
You can set any priorities and weights, wishlist will just list them all.
Then, you can use dig to verify it works, e.g.:

dig -t SRV _ssh._tcp.caarlos0.dev +short
1 1 22 s01.caarlos0.dev.                                                                                                                                                                                                                                             
10 2 2222 s01.caarlos0.dev.

You can also customize the name of the endpoints by adding TXT records to the domain root, e.g.:

dig -t TXT caarlos0.dev +short
"wishlist.name s01.caarlos0.dev:2222=confettysh"

Just make sure to use the format wishlist.name {address}:{port}={name}, and it should magically work.

And, to use it, run wishlist --srv.domain caarlos0.dev.

That's it :)