Add an mDNS client

[willstott101]

Opening this issue to show a client is in-scope for this project, but not yet implemented. This follows a mention in #4 (comment)

Questions to resolve:

• Can we provide any meaningful improvement over the mDNS resolution in https://crates.io/crates/trust-dns-resolver ? Probing before announcing is not implemented #37
• Would a client need to be hidden behind a feature flag? No probing is a core part of the spec.