default ItemSearch max_items to 100, and make its use more prominent

[philvarner]

Related Issue(s): #187

Description:

• default ItemSearch max_items to 100 instead of unlimited, as that could retrieve a huge number of pages and/or items

PR Checklist:

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

[codecov-commenter]

Codecov Report

Merging #208 (7e47f5a) into main (4706765) will decrease coverage by 0.18%.
The diff coverage is 78.26%.

@@            Coverage Diff             @@
##             main     #208      +/-   ##
==========================================
- Coverage   83.93%   83.75%   -0.19%     
==========================================
  Files           9        9              
  Lines         660      677      +17     
==========================================
+ Hits          554      567      +13     
- Misses        106      110       +4     

Impacted Files	Coverage Δ
pystac_client/client.py	80.00% <0.00%> (ø)
pystac_client/item_search.py	91.96% <80.95%> (-0.90%)	⬇️
pystac_client/collection_client.py	76.74% <100.00%> (ø)

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