Add support for variant constraint values to PDL packet declarations

Example use case taken from the UWB's UCI specification:
```
enum MessageType: 3 {
DATA = 0x00,
COMMAND = 0x01,
RESPONSE = 0x02,
NOTIFICATION = 0x03,
}

packet UciPacket {
message_type: MessageType,
_body_
}

packet UciControlPacket: UciPacket (message_type = COMMAND | RESPONSE | NOTIFICATION) {
opcode: 8,
_size_(_payload_): 8,
_payload_,
}

packet UciDataPacket: UciPacket (message_type = DATA) {
_size_(_payload_): 16,
_payload_,
}
```
In this example UciControlPacket covers multiple message types. It is not possible to describe this constraint using the exisiting syntax.