Add `system.db.*` constants to `__all__`

[cesarcoatl]

Is your feature request related to a problem? Please describe.

When I run mypy against incendium I get the following errors regarding system.db constants:

$ mypy src
src/incendium/db.py:166: error: Module has no attribute "INTEGER"
src/incendium/db.py:253: error: Module has no attribute "BIT"

Describe the solution you'd like

In order to make those constants available on ignition-api-stubs, all constants must be added to __all__.

Describe alternatives you've considered

Additional context