Coerce cql2 style to match HTTP method

[jsignell]

Related Issue(s):

• Closes Filter CQL2 style needs to match HTTP method in search #803

Description:

Coerced the filter value to cql2-json for POST and cql2-text for GET unless the user specifies the filter-lang.

This uses cql2-rs python bindings and adds cql2 as a dev dependency, but it fails with a nice error message if that library is not in the env.

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to CHANGELOG.md

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.99%. Comparing base (21435b0) to head (1af0f93).
Report is 125 commits behind head on main.

Files with missing lines	Patch %	Lines
pystac_client/item_search.py	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #804      +/-   ##
==========================================
+ Coverage   93.43%   93.99%   +0.55%     
==========================================
  Files          13       15       +2     
  Lines         990     1232     +242     
==========================================
+ Hits          925     1158     +233     
- Misses         65       74       +9     

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[jsignell]

Looks like the failures are related to this: actions/runner-images#12576