Add support for serde rename_all_fields container attribute

[yu256]

Adds support for serde's rename_all_fields container attribute on enums.

#[derive(Serialize, ToSchema)]
#[serde(rename_all_fields = "camelCase")]
enum MyEnum {
    One { some_number: i32 },       // -> someNumber
    Two { another_field: String },   // -> anotherField
    Unit,                            // unaffected
}

Per-variant #[serde(rename_all = "...")] overrides rename_all_fields, following serde's specification.

Closes #853

[CommanderStorm]

I reviewed the tests nad the code and both LGTM, no comments from my side.

I think the tests cover all aspects (even though those are a bit hard to review given the chore that snapshot tests are 😉)