Support multi Database Health Checks

[HanaPoulpe]

Working with django applications that work across multiple database might require more detailed healthchecks (eg: 1 per database, read only checks)

These feels like a 3 changes:

• Add a factory to create database backends subchecks DatabaseBackend[some_database] that checks RW permissions
• Add a factory to create database backends subchecks DatabaseBackendRO[some_database] that checks RO permissions (useful if the application uses a read replica)
• Add a factory to create migrations subchecks MigrationsHealthCheck[some_database] that checks that migration are applied to a specific database

This approach seems better than running existing checks on all databases by default, as it allows to separate database between what is required for an application as readiness checks and what is required for its liveness checks.

I can take the implementation, as I just had to build something similar on a project I work on.