Add support for the STAC API Collection Search Extension

[juliayun23]

https://github.com/stac-api-extensions/collection-search

I noticed that stac-browser v3.1.0 has added support for Collection Search. We are using stac-browser on top of stac-fastapi.

Just wondering whether there's any plan of adding the support for Collection Search Extension in stac-fastapi? Thanks.

[vincentsarago]

partially done in #736
post extension proposed in #739