Continuing discussion started in Issue #248

The *sqlx.DB reference for plugins is required as when its loaded an immediate call to db.Ping() is made. I can open a separate issue to drop the reference altogether as the plugin should be able to manage the connection on its own and doesn't need the reference it returns to be passed into its own exposed functions as a parameter.

Additionally, it would be great to extend plugins for not only database backends but for signing mechanisms as well (e.g. for HashiCorp Vault or GCP IAM API).

Finally, I also propose to enhance the system in general and export minor parts of the bootstrap process for those of us that want to customize hydra directly vs loading a plugin.