Register database engines directly from db lib

[greedy52]

This is a follow-up of

• Fix issue "redis" engine is not registered #19239

As suggested by #19239 (comment), srv/lib now explicitly register the database engines, instead of relying on importing the "engine" packages.

Also added a check to confirm engines are registered when creating a database service. This helps capture the problem in test and integration test

ERROR REPORT:
Original Error: *trace.NotFoundError database engine "sqlserver" is not registered
Stack Trace:
	/Users/stevehuang/go/github.com/gravitational/teleport/lib/srv/db/common/engines.go:73 github.com/gravitational/teleport/lib/srv/db/common.CheckEngines
	/Users/stevehuang/go/github.com/gravitational/teleport/lib/srv/db/server.go:305 github.com/gravitational/teleport/lib/srv/db.New
	/Users/stevehuang/go/github.com/gravitational/teleport/lib/service/db.go:134 github.com/gravitational/teleport/lib/service.(*TeleportProcess).initDatabaseService
	/Users/stevehuang/go/github.com/gravitational/teleport/lib/service/supervisor.go:539 github.com/gravitational/teleport/lib/service.(*LocalService).Serve
	/Users/stevehuang/go/github.com/gravitational/teleport/lib/service/supervisor.go:276 github.com/gravitational/teleport/lib/service.(*LocalSupervisor).serve.func1

[timothyb89]

Definitely nice to see init functions go away