Make CDnsSeedOpts unit testable

Summary:
Move CDnsSeedOpts to its own files to make it more testable.
Only one basic proof of concept test was added to not clutter this patch.
Better test coverage will come with improved argument checking (example: D10748)

Test Plan:

ninja check-seeder bitcoin-seeder

Sanity checks:

./src/seeder/bitcoin-seeder -host=seeder.status.cash -ns=localhost -mbox=thonkle@protonmail.com -port=5555
less dnsseed.dump

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10790