remove `post_request_model` attribute in the BaseCoreClient and AsyncBaseCoreClient

[vincentsarago]

stac-fastapi/stac_fastapi/types/stac_fastapi/types/core.py

Line 344 in 753341f

post_request_model = attr.ib(default=BaseSearchPostRequest)

This attribute is not used in the abstract methods and is not event guarantee to be used in the client. We should let backend define what attribute they need

cc @hrodmn