Traverse through catalogs

[m-mohr]

Like stac-validator, it would be great to specify a STAC entity and traverse from it through all links and validate them.

[m-mohr]

Maybe re-use the crawler code from stac-index, which might include a "sleep" time between requests, a queue for retries and sampling just a number of items instead of reading them all.

cc @DanielJDufour

[m-mohr]

It can at least work on full folders now. This is not really crawling though and may look into JSON files that are not STAC.