fix(config): Use correct Deserialize impl for enums (#389)

Since the latest version of `schematic`, untagged enums have a custom
`Deserialize` implementation that conflicts with our `with` attributes.
The new `skip_custom_untagged_enum_deserialize_impl` attribute ensures
our `with` attributes are used instead.

Additionally, the new `literal` attribute makes sure that `schematic`'s
schema generation uses string literals for enum variants, as opposed to
the default `null` for unit variants in enums.

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Signed-off-by: Jean Mertz <git@jeanmertz.com>

Author: JeanMertz