Add OpenAPI MIME and rel types

[duckontheweb]

Adds "service-desc" rel type and "application/vnd.oai.openapi+json;version=3.0" MIME type for OpenAPI links in STAC API.

[codecov-commenter]

Codecov Report

Merging #101 (5ff1318) into master (41f0161) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   96.19%   96.21%   +0.01%     
==========================================
  Files          20       20              
  Lines         473      475       +2     
==========================================
+ Hits          455      457       +2     
  Misses         18       18              

Flag	Coverage Δ
unittests	96.21% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
stac_pydantic/links.py	98.24% <100.00%> (+0.03%)	⬆️
stac_pydantic/shared.py	100.00% <100.00%> (ø)

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