Extension types for ACP protocol extensibility.

ACP reserves method names prefixed with _ (e.g. _myext/doThing) for vendor/experimental extensions that aren't part of the core spec. Peers that don't understand an extension method still parse it uniformly:

• Ext.ExtRequest — an arbitrary _-prefixed request (expects a reply).
• Ext.ExtResponse — the reply to an ExtRequest. Its wire representation is transparent: the response body IS the wrapped data term, not %{"data" => ...} — see the module doc there.
• Ext.ExtNotification — an arbitrary _-prefixed notification (no reply expected).

Ported from the MIT f1729/agent_client_protocol (c) 2025 f1729; see NOTICE.md.