feat: add custom header support to PHT protocol

- Add Header field to PHT Client and clientConn structs
- Support custom headers in authorize, push, and pull requests
- Add metadata parsing for header configuration as map[string]string
- Enables PHT usage with header-based authentication (e.g., Cloudflare Access)

Backwards compatible - headers are optional.

dialer/pht/dialer.go

@@ -109,6 +109,7 @@ func (d *phtDialer) Dial(ctx context.Context, addr string, opts ...dialer.DialOp
 			PushPath:      d.md.pushPath,
 			PullPath:      d.md.pullPath,
 			TLSEnabled:    d.tlsEnabled,
+			Header:        d.md.header,
 			Logger:        d.options.Logger,
 		}
 		d.clients[addr] = client