Add interfaces to rust/ekore

[felixhekhorn]

• Add C/C++ interface
• Test C/C++ interface in CI
• Document C/C++ interface
• Repeat for Fortran

[felixhekhorn]

Question: do we want to expose a wrapper for

eko/src/eko/evolution_operator/__init__.py

Lines 390 to 401 in 0287086

	if is_polarized:
	if is_time_like:
	raise NotImplementedError("Polarized, time-like is not implemented")
	else:
	gamma_singlet = ad_ps.gamma_singlet(order, ker_base.n, nf)
	else:
	if is_time_like:
	gamma_singlet = ad_ut.gamma_singlet(order, ker_base.n, nf)
	else:
	gamma_singlet = ad_us.gamma_singlet(
	order, ker_base.n, nf, n3lo_ad_variation, use_fhmruvv
	)

or are we fine with just exposing directly ad.u.s.gamma_ns_qcd and friends and leave the user with the if branches?