Add experimental ACP support to coder/agentapi

[johnstcn]

To start validating support for ACP with Coder Tasks, we want to start by adding (experimental) ACP support in coder/agentapi.

Acceptance criteria:

• Running agentapi server --experimental-acp -- <acp-enabled-agent> should work as expected for send/recv messages via REST API, attach, and web UI.