fix sortby typing and format for POST

[philvarner]

Related Issue(s): #155

Description:

• I'm pretty sure using sortby with POST would just fail on a compliant implementation. Fixed now.
• Updated typing to be correct wrt GET and POST formats
• search now accepts either a string, list[str], or dict for sortby, and converts appropriately for GET or POST
• added tests for sortby

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to the CHANGELOG

[codecov-commenter]

Codecov Report

Merging #175 (dc0705a) into main (28adaf1) will increase coverage by 1.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
+ Coverage   81.39%   82.60%   +1.21%     
==========================================
  Files           9        9              
  Lines         575      592      +17     
==========================================
+ Hits          468      489      +21     
+ Misses        107      103       -4     

Impacted Files	Coverage Δ
pystac_client/item_search.py	90.74% <100.00%> (+2.20%)	⬆️

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