Add AVAILABLE_DATABASES registry for database discovery

[felipeadeildo]

Closes #234

The changes includes:

• Add AVAILABLE_DATABASES list to pysus namespace containing all 9 database classes
• Replace wildcard imports with explicit imports in pysus.ftp.databases.__init__.py
• Add a test suite (9 tests) for database registry functionality
• Update README with usage example

Users can now programmatically discover all available databases:

from pysus import AVAILABLE_DATABASES

for db_cls in AVAILABLE_DATABASES:
    db = db_cls()
    print(f"{db.name}: {db.metadata['long_name']}")