Replace GET with POST and update unit tests

[jan-bausch]

This replaces calls to the Graphhopper directions API from GET to POST.

I've also updated the corresponding unit tests.

[jan-bausch]

I converted params into a dictionary, pluralized the relevant parameters and updated the corresponding unit test

Needs further work, depending on https://github.com/gis-ops/routingpy/pull/91#discussion_r1141971247

[jan-bausch]

I've updated a handful of arguments to be identical with the docs:

Renamed point_hint to point_hints, heading to headings, snap_prevention to snap_preventions, curb_side to curbsides,

Added custom_model parameter

Removed weighting, block_area, avoid, turn_costs parameters

[chrstnbwnkl]

Thanks, really appreciating help on this, hard to always keep up with all the API changes :)

[nilsnolde]

Ah damn, forgot that we have a changelog :sweat_smile: We'll get it before we release (likely this week)

[jan-bausch]

Awesome, thanks for the quick feedback 🎉