Add custom resolver support to Client

[eljeffeg]

Closes #85.

Summary

• add a public Resolver trait and closure implementation;
• add Client::with_resolver while preserving the existing default DNS behavior;
• use resolver-returned addresses directly for the connection while retaining the URI hostname for Host and TLS verification;
• invoke the resolver for every followed redirect;
• retain bad-port validation and reject empty resolver results;
• add unit coverage for custom resolution, empty results, bad-port enforcement, and redirects.

This supplies the transport extension point needed by applications with custom DNS or outbound destination policies without putting an application-specific SSRF policy into oxhttp itself.

Validation

• cargo fmt --check
• cargo clippy --all-targets --all-features -- -D warnings -D clippy::all
• cargo test --all-features (60 unit tests and 6 documentation tests)