Split default IO down into to/from href methods

[gadomski]

Related Issue(s): stac-utils/stactools#113

Description: This allows subclasses of DefaultStacIO to not have to re-impelment link resolution if they don't want to. I would like this for, e.g., this fsspec io, where right now I'm just copying the link resolution code from pystac.

PR Checklist:

• Code is formatted (run scripts/format)
• Tests pass (run scripts/test)
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #354 (8cedba5) into main (61f62b1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #354   +/-   ##
=======================================
  Coverage   88.58%   88.59%           
=======================================
  Files          36       36           
  Lines        4625     4629    +4     
=======================================
+ Hits         4097     4101    +4     
  Misses        528      528           

Impacted Files	Coverage Δ
pystac/stac_io.py	72.38% <100.00%> (+0.84%)	⬆️

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