Allow additional parameters via a "public" interface

[gadomski]

Currently, a user could modify _parameters on an item search to add other parameters to a request:

pystac-client/pystac_client/item_search.py

Lines 312 to 318 in ffe964f

	def get_parameters(self) -> Dict[str, Any]:
	if self.method == "POST":
	return self._parameters
	elif self.method == "GET":
	return self._clean_params_for_get_request()
	else:
	raise Exception(f"Unsupported method {self.method}")

We should add a "public" interface to this attribute, probably set_parameter(key, value). One use-case would be to test pystac-client with new/proposed API extensions without needed to touch the "private" interface or modify pystac-client itself.