[ActiveDispatch] RequestId Middleware: Allow configuration of header name

Hi,

Currently, the RequestId middleware assumes that the request’s ID is delivered in the X-Request-Id HTTP header. The value is hardcoded (see here). However, in our usecase, a different header name is used that we have no control over. So, it seems useful to be able to configure the header name to support environments that use other conventions for propagating a request ID.

Does this sound like a useful enhancement? Any opinions?