connector: hex-decode mtproxy secret

dcs.MTProxy expects raw secret bytes. Carrying them verbatim through a
YAML string field is impossible: real secrets contain bytes >= 0x80
(faketls starts with 0xee, secured with 0xdd) which cannot survive a
unicode string round-trip, so the value reached the bridge corrupted or
empty (gotd then logged "invalid secret").

Accept the standard hex form printed by mtg/MTProxy tooling
(e.g. "ee" + 16-byte secret + cloak domain hex) and decode it before
handing the bytes to gotd.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pkg/connector/example-config.yaml

@@ -61,6 +61,8 @@ proxy:
     # Proxy IP address/domain name and port.
     address: "127.0.0.1:1080"
     # Proxy authentication (optional). Put MTProxy secret in password field.
+    # For mtproxy, the secret must be hex-encoded (the same form mtg/MTProxy
+    # tools print, e.g. "ee" + 16-byte secret + cloak domain hex for faketls).
     username:
     password: