chore: generalise database management

[gruyaume]

Description

This PR does a couple of things related to the database. Those are mainly because the database is no only used for certificate management anymore, it also stores users and soon Certificate Authority content.

• Rename the file from certdb to db
• Treat the certificateRequest and users table in the same way. Both are now un-parametrized and variables are used to referenced to both table names.
• Rename CertificateRequestsRepository to Database to reflect that the db contains varying types of data.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules