Allow custom models for search GET and items endpoints

[lossyrob]

Description:
I use a different default limit than this project, which is 10. While currently you can override the search_request_model in the StacApi class, you can't do so for the models that give default values to the item collection endpoint and the GET search. This PR adds models that allow users to override these types. It also contains the ability for the pgstac backend to use a custom search model instead of building a PgstacSearch for the item collection and GET search endpoints. I made this change while tracking down why my overridden limit was not being used; this turns out not being needed for my specific use case (as the default limit comes from the arguments passed into the core client methods by ItemCollectionUri and SearchGetRequest), it seemed like a sensible change in case any other user would want to modify the behavior of the search model in more than just the search POST endpoint.

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• 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.

[moradology]

This looks good. There's some duplication of arguments but the fix (described in a below comment) might not be desirable. +1 if the duplication fix isn't something we want to pursue

[moradology]

It might be possible to avoid this duplication of search_request_model specification if the client methods that use it were higher order functions. This would mean taking the StacApi specified search_request_model and returning a function which would then be registered as an endpoint.

[lossyrob]

Interesting point, perhaps a more broad pattern that could be applied in follow up work, but would likely take a larger refactor that may or may not be worth it.