support custom handler while still supporting easy to use collector

[GlenDC]

The enum Collector provided by this library is easy to use and works out of the box.
However it does not support any specialised custom handler logic that might be needed for some use cases.

This issue (feature request) flows out of #16. Turns out that the handler allows this kind of feature and more.
However I feel that adding that support to the collector directly overcomplicates stuff for the benefit of only some users (me).

So therefore I propose that the client instead works with a generic collector, so that by default one can still use
the Collector but can also implement their own if they wish any specialised handler logic (e.g. SSL modifications, header reads, byte tracking etc...).