Use real rust type for pallet alias in `runtime` macro (#4769)

Fixes https://github.com/paritytech/polkadot-sdk/issues/4723. Also,
closes https://github.com/paritytech/polkadot-sdk/issues/4622

As stated in the linked issue, this PR adds the ability to use a real
rust type for pallet alias in the new `runtime` macro:
```rust
#[runtime::pallet_index(0)]
pub type System = frame_system::Pallet<Runtime>;
```

Please note that the current syntax still continues to be supported.

CC: @shawntabrizi @Kianenigma



---------

Co-authored-by: command-bot <>
Co-authored-by: Bastian Köcher <[email protected]>