Provisioners and DB engines registry

[maraino]

Description

Currently, KMSs are registered in certificates when the file is imported using the apiv1.Register method, for example, cloudkms is registered using this:

certificates/kms/cloudkms/cloudkms.go

Lines 96 to 100 in 3f660ff

	func init() {
	apiv1.Register(apiv1.CloudKMS, func(ctx context.Context, opts apiv1.Options) (apiv1.KeyManager, error) {
	return New(ctx, opts)
	})
	}

Then we import the supported kms engines in main.go:

certificates/cmd/step-ca/main.go

Lines 27 to 33 in 3f660ff

	// Enabled kms interfaces.
	_ "github.com/smallstep/certificates/kms/awskms"
	_ "github.com/smallstep/certificates/kms/cloudkms"
	_ "github.com/smallstep/certificates/kms/softkms"
	// Experimental kms interfaces.
	_ "github.com/smallstep/certificates/kms/yubikey"

This allows third-party packages to import some of our stuff without the need to import extra libraries or unused functionality.

We should do the same for provisioners and DB engines. Doing this, it will make it easier to extend step-ca with new provisioners, and new DB engines. It would be also possible to create specific builds that enable only specific things.