REFACTOR: Add diff2 enable flag to all DNS providers (#1851)

This should enable the diff2 code to be inserted with good "git blame" results for new code.  I'm adding this early to catch any problems early.

commands/commands.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"github.com/StackExchange/dnscontrol/v3/models"
+	"github.com/StackExchange/dnscontrol/v3/pkg/diff2"
 	"github.com/StackExchange/dnscontrol/v3/pkg/js"
 	"github.com/StackExchange/dnscontrol/v3/pkg/printer"
 	"github.com/urfave/cli/v2"
@@ -57,6 +58,11 @@ func Run(v string) int {
 			Usage:       "Enable JS fetch(), dangerous on untrusted code!",
 			Destination: &js.EnableFetch,
 		},
+		&cli.BoolFlag{
+			Name:        "diff2",
+			Usage:       "Enable replacement diff algorithm",
+			Destination: &diff2.EnableDiff2,
+		},
 	}
 	sort.Sort(cli.CommandsByName(commands))
 	app.Commands = commands