Support session-usage in copilot --acp

### Describe the feature or problem you'd like to solve

Session usage will give more visibility into the session context (used tokens, cost, etc)

### Proposed solution

The proposal is to implement the existing RFD: https://agentclientprotocol.com/rfds/session-usage

While the RFD is in draft, this is a very important feature for users. Furthermore, early adoption will help validate the RFD or even allow the Copilot CLI team to find early incompatibilities (especially because Copilot measures "Premium requests", which is a bit different than what others do, so you probably want to ensure whatever ends up in the standard can handle your structure).

### Example prompts or workflows

_No response_

### Additional context

_No response_