Add OpenAPI document

[iliion]

Hi. I cannot find any specification for the collection-search https://api.stacspec.org/v1.0.0-rc.1/collection-search

[m-mohr]

API conformance classes often do not resolve and don't need to resolve.

The documentation is here: https://github.com/stac-api-extensions/collection-search

An OpenAPI document is missing, PRs are welcome.