Refactor the library to allow database-specific extensions

[andrewslotin]

The current architecture of the extension requires adding each database driver as a dependency, which does not scale well, as adding support for a new database creates the demand for keeping its driver up-to-date and increases the risk of introducing vulnerabilities to the extension code. Adding the db driver as a dependency is needed to make the sql.Open() work due to the design of Go's stdlib package.

As an alternative solution we propose to redesign the API of this extension to allow users to use it as a base for database-specific extensions that can be used depending on the use-case.