feat: MVP

[hrodmn]

I have been wanting to try this out for a while! The goal is to create a legit collection-search-only STAC API that hits the /collections endpoint on multiple APIs and returns the concatenated results. It's not very safe yet (still need to add a check that the collection-search and free-text extensions are enabled) but it works!

TODO:

• add tests
• add docker-compose.yml with stac-browser

ref: developmentseed/federated-collection-discovery#102

[gadomski]

Makes a ton of sense to me, and gets us closer to the "One STAC to rule them all."

I'm wondering if we even need to pre-configure the child APIs? Could we allow request.app.state.settings.child_api_urls to be overriden by the request?

[gadomski]

Nit: we can just use tuple and dict as of Python 3.10.

Suggested change

	) -> Tuple[str, Dict[str, Any]]:
	) -> tuple[str, dict[str, Any]]: