Describe the bug

When attempting to define astac-fastapi-core-based application with the Query Extension disabled, I found that stac-fastapi-core would emit an attribute error and crashfail.
Specifically, the logic for handling a POST Search request here:

I found that this also happens with the sortby and token pagination extensions. Removing these makes the application inoperable.

To Reproduce

1. Create a new application, e.g., based off of the mongodb example
2. Remove the QueryExtension from the application's extensions
3. Start the app (it will start)
4. Send any valid POST Search
5. 💥

Expected behavior

With Query disabled, searches should function.
The same holds for Sort and Token Pagination, but the case is clearest with Query (which is deprecated).

Additional context

I'm experimenting with building an application for my team, so I don't have a lot of knowledge around STAC. We're still learning the spec(s) and trying things out.
For now, we're trying to get a very early stage prototype up and running, so I've implemented this very clumsy hack:

classCustomizedCoreClient(CoreClient):asyncdefpost_search(self,search_request,request):object.__setattr__(search_request,"query",None)object.__setattr__(search_request,"sortby",None)object.__setattr__(search_request,"token",None)returnawaitsuper().post_search(search_request,request)

I think the best pattern for the stac-fastapi-core code would be to check withhasattr(...) or three-argumentgetattr(..., ..., None).

I'm happy to contribute a PR with this sort of fix if it would be welcome.