Add raw request body to middleware.Request for signature validation use cases

This PR adds a new field RawBody []byte to the middleware.Request struct, allowing middleware to access the raw HTTP request body.

Motivation

In some scenarios — such as validating HMAC or SHA-based signatures — it is necessary to work with the exact raw body of the incoming request. Currently, ogen consumes the request body during decoding, and does not expose it to middleware.

Example

Demo project here

Update:

I found an existing PR that introduces a similar idea: #1505, where the author recovers the body using io.NopCloser.

The goal is the same - to have access to the original body.

Compared to that approach, this PR has the following advantages:
• No need to manually defer r.Body.Close() or wrap the request body with io.NopCloser.
• Middleware receives the raw body directly via middleware.Request.RawBody, so there’s no need to re-read or re-wrap the body again.

This approach keeps the original request lifecycle untouched and adds the raw body as a read-only byte slice for middleware purposes.