add Grid Extension support

[philvarner]

Related Issue(s):

• Create Grid Extension from code in stactools sentinel2 package #783

Description:

Add Grid Extension support https://github.com/stac-extensions/grid/

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• 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.

[duckontheweb]

Thanks for this @philvarner . All of the extension-related changes look good to me, I just had a question regarding whether we want to add the pytest configuration as part of this PR.