Update the /get_allowed_conflations endpoint

[gaurav]

We have a /get_allowed_conflations endpoints that is currently hardcoded to return GeneProtein. I think the plan was to generalize the conflation features (#130) so that we could return a set of codes that could be used to turn on various conflations (e.g. /get_normalized_nodes?conflation=GeneProtein). Once we close #130 this won't be needed, but maybe until then we should remove this endpoint or add DrugChemical to the list?

Endpoint: https://nodenormalization-sri.renci.org/docs#/default/get_conflations_get_allowed_conflations_get

NodeNormalization/node_normalizer/server.py

Lines 162 to 174 in 02574e7

	@app.get(
	"/get_allowed_conflations",
	summary="Get the available conflations",
	description="The returned strings can be included in an option to /get_normalized_nodes",
	)
	async def get_conflations() -> ConflationList:
	"""
	Get implemented conflations
	"""
	# TODO: build from config instead of hard-coding.
	conflations = ConflationList(conflations=["GeneProtein"])
	return conflations