Support for request body parsing

As far as I can see, `phr_parse_request` has the ability to parse the request headers (+method, url and http version), but not the body, which is essential for POST-requests. It would be really nice, if the parser has an option to extract and return the body

Start at the `phr_parse_request` entry point and review how it currently parses request headers, method, URL, and HTTP version. Determine the API and parsing behavior needed to expose a request body for POST requests; done means the parser can optionally extract and return that body without losing existing header parsing behavior.

Written by the indexing model from the issue text.