Feature/add collection search extension

[tjellicoe-tpzuk]

Related Issue(s): #615

Description: Adding Collection Search Extension as defined here

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.

[m-mohr]

This doesn't quite follow https://github.com/stac-api-extensions/collection-search

The extension doesn't define a POST /collections endpoint, only GET /collections. It doesn't define a new endpoint, it adds to the existing GET /collection.

All functionality in OGC API - Records - Part 1: Local Resource Catalogue is only defined for the GET method (i.e. GET /collections). Note: STAC may add behavior for POST /collections in the future, but due to a potential conflict with the Transaction Extension, specific rules for content negotiation might be required.