set filter-lang parameter correctly based on the filter parameter type

[philvarner]

Related Issue(s): #149

Description:

Set filter-lang to cql2-text if the filter paramater is a string, set it to cql2-json if it's a dict.

PR Checklist:

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

[codecov-commenter]

Codecov Report

Merging #169 (58f7fc6) into main (dbdc3a4) will decrease coverage by 0.00%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##             main     #169      +/-   ##
==========================================
- Coverage   81.00%   80.99%   -0.01%     
==========================================
  Files           9        9              
  Lines         558      563       +5     
==========================================
+ Hits          452      456       +4     
- Misses        106      107       +1     

Impacted Files	Coverage Δ
pystac_client/item_search.py	88.16% <91.66%> (-0.18%)	⬇️
pystac_client/client.py	78.68% <100.00%> (ø)

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