Add simple version of get_queryables

[jsignell]

Related Issue(s):

• closes Add support for fetching catalog queryables #231

Description:

I tried to implement this as a QueryableMixin class but the types were convoluted enough that I ended up thinking it was clearer to just put the method on each class.

PR Checklist:

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

[codecov-commenter]

Codecov Report

Patch coverage: 90.74% and project coverage change: +0.02 🎉

Comparison is base (8786fcf) 87.27% compared to head (56fc9b6) 87.29%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #477      +/-   ##
==========================================
+ Coverage   87.27%   87.29%   +0.02%     
==========================================
  Files          11       12       +1     
  Lines         833      858      +25     
==========================================
+ Hits          727      749      +22     
- Misses        106      109       +3     

Impacted Files	Coverage Δ
pystac_client/collection_client.py	87.09% <88.00%> (-1.80%)	⬇️
pystac_client/mixins.py	92.00% <92.00%> (ø)
pystac_client/client.py	81.96% <100.00%> (+0.14%)	⬆️

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[gadomski]

In general, I like the mixin pattern, and StacAPIIO will give us a nice place to put all href resolution in #480.