cherry-pick: 4166 udp upstream

Merge in DNS/adguard-home from 4166-udp-upstream to master

Closes #4166.

Squashed commit of the following:

commit b8b6d1c7ac1e11e83c0c68e46e7f66fdc6043839
Merge: e5f01273 ea6e033d
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Mar 1 20:36:40 2022 +0300

    Merge branch 'master' into 4166-udp-upstream

commit e5f0127384d84c4395da5b79a1fd4a47acbe122c
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Mar 1 19:41:33 2022 +0300

    client: upd upstream examples

commit bd974f22231f11f4c57e19d6d13bc45dbfdf2fdf
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Mar 1 18:36:10 2022 +0300

    all: upd proxy

commit badf1325090ecd1dc86e42e7406dfb6653e07bf1
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Feb 4 14:36:50 2022 +0300

    WIP

internal/dnsforward/http.go

@@ -404,7 +404,7 @@ func ValidateUpstreams(upstreams []string) (err error) {
 	return nil
 }
 
-var protocols = []string{"tls://", "https://", "tcp://", "sdns://", "quic://"}
+var protocols = []string{"udp://", "tcp://", "tls://", "https://", "sdns://", "quic://"}
 
 func validateUpstream(u string) (useDefault bool, err error) {
 	// Check if the user tries to specify upstream for domain.

internal/dnsforward/http_test.go

@@ -303,6 +303,14 @@ func TestValidateUpstream(t *testing.T) {
 		name:     "valid_default",
 		upstream: "sdns://AQMAAAAAAAAAFDE3Ni4xMDMuMTMwLjEzMDo1NDQzINErR_JS3PLCu_iZEIbq95zkSV2LFsigxDIuUso_OQhzIjIuZG5zY3J5cHQuZGVmYXVsdC5uczEuYWRndWFyZC5jb20",
 		wantErr:  ``,
+	}, {
+		wantDef:  assert.True,
+		name:     "default_udp_host",
+		upstream: "udp://dns.google",
+	}, {
+		wantDef:  assert.True,
+		name:     "default_udp_ip",
+		upstream: "udp://8.8.8.8",
 	}, {
 		wantDef:  assert.False,
 		name:     "valid",