Merge Capabilities and RESTCapabilities

[soxofaan]

This came up while looking into #610:

openeo-python-client/openeo/capabilities.py

Lines 8 to 12 in 35b5fc7

	# TODO Is this base class (still) useful?
	class Capabilities(ABC):
	"""Represents capabilities of a connection / back end."""

in python client we have openeo.capabilities.Capabilities and openeo.capabilities.rest.RESTCapabilities. This still stems from initial 2018-ish design of the openeo package, but I don't see the point of the abstraction here. I think we're better off with just a single Capabilities implementation.

[soxofaan]

started with a draft PR #613