Suport oneOf as a rust struct enum.

Details:

Enum without a discriminator is untagged being
"untagged" simply means serde won't attempt to
store the name of the enum inside the serialized
object. See
https://serde.rs/enum-representations.html#untagged
for more

Also check to make sure the mapping values
are not an empty object (aka null).

Co-authored-by: Nikita Puzankov <[email protected]>

No longer needed as of reqwest 0.10, it now takes the response as owned instead of mut ref.

Is not empty is more clear

Will show as a struct enum when there are additional fields, otherwise will be a tuple enum.

not sure the purpose of x-mapped-models, perhaps legacy code? mappedModels appears to do the same thing.

Also add default implementation for quality of life

…f24/openapi-generator into ctrlaltf24-basic-rust-anyof-support