tailcfg, ipn/ipnlocal: add Hostinfo.SSH_HostKeys, send when SSH enabled

(The name SSH_HostKeys is bad but SSHHostKeys is worse.)

Updates #3802

Change-Id: I2a889019c9e8b065b668dd58140db4fcab868a91
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tailcfg/tailcfg_test.go

@@ -32,7 +32,7 @@ func TestHostinfoEqual(t *testing.T) {
 		"ShieldsUp", "ShareeNode",
 		"GoArch",
 		"RoutableIPs", "RequestTags",
-		"Services", "NetInfo",
+		"Services", "NetInfo", "SSH_HostKeys",
 	}
 	if have := fieldsOf(reflect.TypeOf(Hostinfo{})); !reflect.DeepEqual(have, hiHandles) {
 		t.Errorf("Hostinfo.Equal check might be out of sync\nfields: %q\nhandled: %q\n",
@@ -181,6 +181,11 @@ func TestHostinfoEqual(t *testing.T) {
 			&Hostinfo{},
 			false,
 		},
+		{
+			&Hostinfo{SSH_HostKeys: []string{"ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIO.... root@bar"}},
+			&Hostinfo{},
+			false,
+		},
 	}
 	for i, tt := range tests {
 		got := tt.a.Equal(tt.b)