make use of BulkTransactionsClient easily overridable

[keul]

TransactionsClient now exposes a bulk_client_cls, easily customizable in subclasses.

Description:

Currently is quite easy to extend the stac_fastapi.sqlalchemy packageto be connects to custom database structure.
This can be done by OOP inheritance and is working well.

One of the customisation I did in my project is the use of custom TransactionsClient and BulkTransactionsClient. 90% of the original code is OK, I just need to change some class-levels attributes (new ORM models, etc).

Then I found that TransactionsClient is internally using an hardcoded reference to BulkTransactionsClient.

stac-fastapi/stac_fastapi/sqlalchemy/stac_fastapi/sqlalchemy/transactions.py

Line 49 in 4a5d72f

bulk_client = BulkTransactionsClient(session=self.session)

This force me to define the create_item in my subclass.

The PR is just making this reference easier to be overridden.

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.

[gadomski]

Thanks for the contribution, but with #555 merged, please re-open this PR on https://github.com/stac-utils/stac-fastapi-sqlalchemy/. For more background on the backend breakout process, see #517.