Initial implementation of global default SNI option (#3047)

* add global default sni

* fixed grammar

* httpcaddyfile: Reduce some duplicated code

* Um, re-commit already-committed commit, I guess? (sigh)

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

modules/caddyhttp/caddyhttp.go

@@ -29,6 +29,7 @@ import (
 
 	"github.com/caddyserver/caddy/v2"
 	"github.com/lucas-clemente/quic-go/http3"
+	"github.com/mholt/certmagic"
 	"go.uber.org/zap"
 )
 
@@ -111,6 +112,10 @@ type App struct {
 	// affect functionality.
 	Servers map[string]*Server `json:"servers,omitempty"`
 
+	// DefaultSNI if set configures all certificate lookups to fallback to use
+	// this SNI name if a more specific certificate could not be found
+	DefaultSNI string `json:"default_sni,omitempty"`
+
 	servers     []*http.Server
 	h3servers   []*http3.Server
 	h3listeners []net.PacketConn
@@ -134,6 +139,8 @@ func (app *App) Provision(ctx caddy.Context) error {
 
 	repl := caddy.NewReplacer()
 
+	certmagic.Default.DefaultServerName = app.DefaultSNI
+
 	// this provisions the matchers for each route,
 	// and prepares auto HTTP->HTTP redirects, and
 	// is required before we provision each server