Add support for schema_with custon fn reference

[juhaku]

Add support for schema_with custom fn reference that can be used alternatively via builders to alter the compile time schema if schema attributes are not enough nor does not fit to the purpose.

Implement schema_with for ToSchema and IntoParmas named struct fields.

Supported syntax:

fn custom_type() -> Object {
    ObjectBuilder::new()
        .schema_type(utoipa::openapi::SchemaType::String)
        .format(Some(utoipa::openapi::SchemaFormat::Custom(
            "email".to_string(),
        )))
        .description(Some("this is email"))
        .build()
}

#[derive(ToSchema)]
struct Value {
    #[schema(with_schema = custom_type)]
    email: String,
}

#[derive(IntoParams)]
struct Query {
    #[param(with_schema = custom_type)]
    email: String,
}

Resolves #195