Fixes https://github.com/paritytech/polkadot-sdk/issues/1839
Currently, `composite_enum`s do not support pallet instances. This PR
allows the following:
```rust
#[pallet::composite_enum]
pub enum HoldReason<I: 'static = ()> {
SomeHoldReason
}
```
### Todo
- [x] UI Test
* globally upgrade quote to latest 1.0.x (1.0.26)
* globally upgrade syn to final 1.0.x version (1.0.109)
* globally upgrade proc-macro2 to 1.0.56
* upgrade to syn v2.0.13 and fix everything except NestedMeta
* fix parse nested metadata code in decl_runtime_apis.rs
* Port more stuff to syn 2.0
* Make the rest compile
* Ignore error
* update to syn 2.0.14
---------
Co-authored-by: Bastian Köcher <info@kchr.de>
* Implement #[pallet::hold_reason]
* Appease clippy
* cargo fmt
* Update test expectations
* Update test expectations
* Support composite_enum attribute instead
* Update test expectations
* Change hold_reason to composite_enum
* Add UI test for unsupported identifier when using composite_enum
* Fix comment
* Add documentation for pallet::composable_enum
* More docs
* cargo fmt