Refactor database handler initialization with factory function

[MPoppinga]

• Added get_db_handler() factory function in db_handler/__init__.py
• Updated add_to_cache.py and monitor_cache_queue.py to use new factory function
• Simplified database handler creation and import logic
• Improved error handling for unsupported database backends

PR Overview

This PR refactors database handler initialization by introducing a factory function to simplify and unify the creation of database handlers. The changes include replacing direct instantiation with the new get_db_handler factory, updating error handling for unsupported backends, and streamlining import logic.

Reviewed Changes

File	Description
src/partitioncache/cli/add_to_cache.py	Uses get_db_handler instead of direct class instantiation; removes a db_env_file check.
src/partitioncache/cli/monitor_cache_queue.py	Updates database handler instantiation to use get_db_handler while retaining an environment check.
src/partitioncache/db_handler/init.py	Introduces the get_db_handler factory function for handling different database types.

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

src/partitioncache/cli/add_to_cache.py:68

• [nitpick] Consider aligning the backend naming conventions by using 'postgres' consistently in both the condition and the factory call, to improve clarity and reduce potential confusion.

if args.db_backend == "postgresql":

src/partitioncache/cli/add_to_cache.py:66

• The removal of the db_env_file check in the postgres branch here contrasts with its enforcement in monitor_cache_queue; consider enforcing this check uniformly if the db_env_file input is required for proper initialization.

if args.db_env_file is None: