Filter CQL2 style needs to match HTTP method in search

[jsignell]

While refreshing the CQL2 filtering tutorial (#802) I noticed that the HTTP method that you use in search affects whether you should use cql2-text or cql2-json. GET expects cql2-text and POST expects cql2-json. This leaks implementation details to the user and is hard to explain.

What if instead pystac-client coerced the filter value into the proper version before doing the request? There is something similar going on with sortby for instance.

It looks like pystac-client could use cql2-rs python bindings to make this happen with minimal effort. But it would also probably be straightforward to write a little function for mapping back and forth.

[gadomski]

My bias, but I'm 👍🏼 to the cql-rs python bindings over something like pygeofilter or rolling our own. I find pygeofilter too heavy, and I think the conversion between the two is non-trival enough that a dependency is better than an implementation.