Made use of BulkTransactionsClient easily overridable

[keul]

Description:

Migrated from stac-utils/stac-fastapi#559

Currently is quite easy to extend the stac_fastapi.sqlalchemy package to be connectes 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.

https://github.com/stac-utils/stac-fastapi-sqlalchemy/blob/main/stac_fastapi/sqlalchemy/transactions.py#L49

This force me to re-define the create_item in my subclass to just change one line.

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

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run make test) some tests are failing, but also in the current master
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.