trace: support HTTP 100 Continue

[achille-roussel]

The HTTP protocol has a mechanism for a client to ask a server whether it is willing to accept the request body:

• first the client sends the request header with a Expect header
• then the server responds with a body-less message and the status doe 100 Continue
• the client sends the response body
• the server sends the final response

In this data flow, there is a single request and two responses, which isn't supported by our current implementation in internal/debug/tracing, we should address to support this common idiom of the HTTP protocol.