Artem

Add support for generic structs/enums/etc.

1

`opg` does not support generic structs. ```rust #[derive(Debug, serde::Serialize, opg::OpgModel)] struct TestStruct { inner: T, } #[derive(Debug, serde::Serialize, opg::OpgModel)] struct TestSubStruct1 { a: i32, b: i64, } #[derive(Debug, serde::Serialize, opg::OpgModel)]...

I think it will look like: ```rust #[derive(opg::OpgModel)] struct Struct { #[opg("Some value", enum = "Value1,Value2,Value3")] value: String, } ```

Instead of describing the paths in the describe_api macro, I would like to have some proc-macro. Instead of this: ```rust describe_api! { // ... paths: { ("some" / "path" /...