feat: generic parser

[Phillip9587]

Based on the work by @jonchurch in #606 and previous PRs listed in #22 (comment). Dicussion on this topic happened in #22.

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There is one remaining point we need to figure out:

parse function

This PR in its current state passes the body as Buffer and the charset to the user provided parse function:

function customParse(buffer, charset) {
  // will only work if node.js supports this encoding else the user needs to use iconv-lite or similar libraries 
  const stringContent = buffer.toString(charset) 
  // custom parse method
}

With this approach we loose the iconv-lite decoding and the users would need to do that themselves . Should we instead already decode the body and pass it as string without the charset? Like:

function customParse(decodedBodyAsString) {
  // custom parse method
}

I think the most common use-case for the generic parser will be a custom string parser like parsing csv or yaml. Not some custom binary format. As alternative we could expose an option which switches between passing a Buffer or a already decoded string to the parse function.

cc @ctcpip @jonchurch @UlisesGascon @bjohansebas