Make stac-validator and stac-check optional

[gadomski]

Description: They have some unnecessary required dependencies that pollute downstreams. cc @pjhartzell @jkeifer

PR checklist:

• [x] Code is formatted (run scripts/format).
• [x] Code lints properly (run scripts/lint).
• [x] Tests pass (run scripts/test).
• [x] Documentation has been updated to reflect changes, if applicable.
• [x] Changes are added to the CHANGELOG.