hysteria: also accept "hysteria2" protocol string

UI stores v1 and v2 both as "hysteria" with settings.version, but
inbounds that came in from imports / manual SQL can carry the
literal "hysteria2" string and get silently dropped everywhere we
switch on protocol.

Add Hysteria2 constant + IsHysteria helper, use it in the places
that gate on protocol (sub SQL, getLink, genHysteriaLink, clash
buildProxy, json gen, inbound.go validation, xray AddUser).

Existing "hysteria" inbounds are untouched.

closes #4081

database/model/model_test.go

@@ -0,0 +1,22 @@
+package model
+
+import "testing"
+
+func TestIsHysteria(t *testing.T) {
+	cases := []struct {
+		in   Protocol
+		want bool
+	}{
+		{Hysteria, true},
+		{Hysteria2, true},
+		{VLESS, false},
+		{Shadowsocks, false},
+		{Protocol(""), false},
+		{Protocol("hysteria3"), false},
+	}
+	for _, c := range cases {
+		if got := IsHysteria(c.in); got != c.want {
+			t.Errorf("IsHysteria(%q) = %v, want %v", c.in, got, c.want)
+		}
+	}
+}