Move generation of command for artisan db into connectors

[kewne]

This is a proposal to change the current implementation of DBCommand to call into the database specific connector classes, instead of relying on a mix of switch statements and dynamic method calls.

Motivation

My team uses Amazon RDS with IAM authentication and the pixelvide/laravel-iam-db-auth package.
However, this only works for the app connections.
For php artisan db, Laravel directly builds the mysql command to execute and, in particular, it relies on the password configuration directly, which means there is no way to generate it.

Benefits

• By calling into drivers and letting them handle configuration options, it becomes possible to use authentication mechanisms with short-lived tokens like the IAM auth described above
• Allows adding new drivers that support the artisan db command
• Improves the design by:
  1. Making the registration mechanism consistent between app connections and artisan db
  2. Improving cohesion because all the code that handles driver specific configuration is in each connector
  3. Decouples the db command implementation from each driver's configuration structure

I'm interested in doing this work myself but would like to avoid creating the PR to then have it shot down.