[bwplotka]

Adds dns_sd_configs schema validation, /etc/resolv.conf search path and ndots resolution, record-to-TargetGroup mapping (SRV, A, AAAA, MX, NS), and an async UDP/EDNS0 + TCP fallback DNS wire resolver under feature = "discovery". Part of prometheus/scrape.rs#36.

Stack:

1. discovery: add TargetGroup, StaticConfig, and Prometheus duration helpers #39 discovery: add TargetGroup, StaticConfig, and Prometheus duration helpers
2. discovery/dns: add DNS service discovery (dns_sd_configs) #43 discovery/dns: add DNS service discovery (dns_sd_configs) (this PR)
3. http_config: add Prometheus HTTPClientConfig and reqwest client builder #44 http_config: add Prometheus HTTPClientConfig and reqwest client builder
4. discovery/kubernetes: add pod service discovery (role: pod) #45 discovery/kubernetes: add pod service discovery (role: pod)

Commits in reading order:

1. cfd22c8: discovery/dns: add DnsSdConfig, ResolvConf, and target group mapping
2. 809d4d9: discovery/dns: add async UDP/EDNS0 and TCP wire resolver

Intentional differences from Go

• DnsDiscovery implements a minimal RFC 1035 and RFC 6891 EDNS0 UDP/TCP wire codec directly over tokio::net::{UdpSocket, TcpStream} instead of adding an external DNS dependency equivalent to miekg/dns in discovery/dns/dns.go#L27 and discovery/dns/dns.go#L292. Futures require no Send bounds and run on !Send tokio::task::LocalSet executors.
• DnsDiscovery::discover executes one discovery poll cycle returning Result<Vec<TargetGroup>, DiscoveryError> rather than spawning an internal refresh loop as in discovery/dns/dns.go#L121, leaving scheduling to the caller.