Add local part of an email and email as a principals.

For user certificates, if an email is passed as a principal, include
in the principals list the email, and the local-part. This imitates
the behavior for OIDC provisioners on other provisioners like JWK.

On `step ssh certificate` we will only include them if no principals
are passed using the `--principal` flag.

Fixes #389

command/ssh/proxycommand.go

@@ -129,6 +129,7 @@ func doLoginIfNeeded(ctx *cli.Context, subject string) error {
 		return err
 	}
 
+	// There's not need to sanitize the principal, it should come from ssh.
 	principals := []string{subject}
 
 	// Make sure the validAfter is in the past. It avoids `Certificate