Add support for union types

[mchitre]

Currently PDUs cannot contain union types, where a field may contain different types of nested PDUs depending on the value of another field. This is a common use case in many protocols, and so we should support it.

Support could be added by allowing a declaration of fieldtype(...) for a PDU field, similar to what we do for length(...). The function will return a field type based on inspecting the PDU. If it returns nothing, we wire-encode a field type in the PDU, otherwise we use the returned type to correctly decode the PDU.

[mchitre]

Implemented support for Union and nothing.

Wire-encoding is not supported as there is no way in Julia currently to enumerate union component types in a documented way. Undocumented ways exist, but are likely to break with Julia versions. It is better for users to explicitly define the header type field to avoid this risk.