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: https://github.com/stac-utils/pystac-client/blob/ffe964f43926b740f4d3e21bce413ce952ff3874/pystac_client/item_search.py#L312-L318

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.