Feature/add collection search extension

[tjellicoe-tpzuk]

Related Issue(s): #615

Description: Adding Collection Search Extension as defined here. Including adding spatiotemporal searching to the GET /collections endpoint and adding a POST /collections endpoint with the same functionality. Also adding a limit parameter to specify the maximum number of results returned from the search.
Note, there seems to be an issue with the test_client_openapi test now due to the check for "Collections" in the schema. I am not sure what is causing this issue, so any help would be much appreciated. This means the make test fails on this test, although all other tests pass.
Some other formatting changes look to have been made by the pre-commit hooks.

PR Checklist:

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

[vincentsarago]

@tjellicoe-tpzuk thanks for the updates, as mentioned in #684 (comment) we should try to avoid adding more extension related code either in stac_fastapi.types and stac_fastapi.api.

could you try to refactor this PR and move most of the things to the .extensions submodule? 🙏