Intercept HTTP requests for observability

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

When debugging services using Aspire, I often need to see what HTTP request and responses (including payloads) are being sent between services. Currently, if I configure my application to log HTTP requests then I can see HTTP methods, URIs, and status codes in the logs but they do not expose headers or request & response payloads for me to inspect, and either way this requires bespoke configuration for each service.

### Describe the solution you'd like

I would like to be able to observe the HTTP requests/responses (incl payloads) sent between my services surfaced via the Dashboard, CLI, & MCP interfaces. For the latter two, that will allow LLMs and other tooling to assist with debugging.

We could capture these requests in DCP's proxy and expose them.

### Additional context

_No response_