internal/pipewire: implement Core::Sync

Once again, already entirely supported, the offset is not yet fully verified but makes intuitive sense. Will verify this on future occurrences of the message.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pipewire/header_test.go

@@ -29,6 +29,12 @@ func TestHeader(t *testing.T) {
 			Size:   0x28, Sequence: 2, FileCount: 0,
 		}, nil},
 
+		{"PW_CORE_METHOD_SYNC", []byte(sendmsg00Message03Header), pipewire.Header{
+			ID:     pipewire.PW_ID_CORE,
+			Opcode: pipewire.PW_CORE_METHOD_SYNC,
+			Size:   0x28, Sequence: 3, FileCount: 0,
+		}, nil},
+
 		{"PW_SECURITY_CONTEXT_METHOD_CREATE", []byte{
 			// Id
 			3, 0, 0, 0,