Add PROXYv2 header utilities

Add PROXYv2 header utilities

This commit adds a set of utilities for dealing with PROXYv2 headers,
both parsing and generating them. The code has no dependencies from
the networking code and is (for the most part) a "separate library".

The part responsible for handling incoming PROXYv2 headers is
structured as a state machine which accepts data as input and calls a
callback to notify the upper-level code about the data processing
status.

Such a design, among other things, makes it easy to write a thorough
unit test suite for that, as there are fewer dependencies as well as
will not stand in the way of any changes in the networking code.