feat(validate): validate STAC Collection (TDE-170)

[paulfouquet]

Change Description:

• validate can now validate Collection
• Work around for TDE-206 / Fix string enum serialization stac-utils/pystac#654 / Python enum validation Stranger6667/jsonschema#316

Notes for Testing:

-c or --collection to validate the Collections only
-i or --item to validate the Items only
See README for details

Source Code Documentation Tasks:

• README updated (where applicable)
• CHANGELOG (Unreleased section) updated
• Docstrings / comments included to help explain code

User Documentation Tasks:

• Confluence updated (where applicable)

Testing Tasks:

• Added tests that fail without this change
• All tests are passing in development environment
• Reviewers assigned
• Linked to main issue for ZenHub board