Add support for including `ignorable` types in enum de/ser macros

Add support for including `ignorable` types in enum de/ser macros

An enum implements de/serialization via
`impl_writeable_tlv_based_enum_upgradable` currently cannot contain
an object that only implements `MaybeReadable`.

This solves that by implementing the required blocks for
`ignorable`, opting to return `Ok(None)` in the top-level read in
cases where the inner field read returns `Ok(None)`.