OpenAPI spec gen - support #[serde(flatten)] attr

[imp]

serde allows for flattening of the child/parent structure fields. OpenAPI spec generation, however, ignores this and produces different structure in the doc

[E-gy]

Notes for implementation i guess. But i would like comments if anyone has any.

1. Flattening can only work if both parent and child are of schema_type: Object
2. Flattening can not work if either declares max_properties or min_properties
3. properties, and additional_properties, as well as required and dependent_required iff the child is not nullable, are all and only schema parameters that need to be merged (from child into parent)
4. Serde supports untagged enums flattening. Flattening untagged enum schema into a struct means cloning the parent schema enum variants times into oneOf, then merging schema variants from child into generated parents.

Part 4 is particularly concerning, because as of now,

• rweb does not support deriving schemas for struct enums
• Option enum is generated as a single variant of oneOf marked with nullable. as mentioned already, there is no way of doing it properly in OpenAPI < 3.1. so it will either need a special treatment, or migration to generation of OpenAPI spec version 3.1.