Allow specifying insert method for bulk transactions

[edkeeble]

Description:
We have started adopting the bulk transactions extension for loading data into our pgstac backend, but are blocked by the fact that we occasionally need to re-ingest an updated item. The bulk_item_insert route does not provide a means of specifying alternate data loading methods (e.g. upsert, delsert, etc) and stac-fastapi-pgstac only supports inserts via the create_items function. In the event of a conflict, it returns a ConflictError with no details regarding which of the bulk items caused the conflict, which makes it difficult to correct the issue.

In this PR, I propose adding an optional parameter to the request object for the bulk_item_insert route. This method parameter will allow clients to specify the use of upsert rather than the default insert behavior. I have created a separate PR on stac-fastapi-pgstac to support conditionally using the upsert_items function.

This change should not impact any stac-fastapi backends which do not support alternate loading methods, since they will ignore the optional param.

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.