Use real rust type for pallet alias in runtime macro

[gupnik]

Fixes #4723. Also, closes #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:

#[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

[shawntabrizi]

lgtm high level

[kianenigma]

Loogs good, except the doc for #[runtime] macro are not updated.

I also wanted to log this for you in a new issue that I have not done so far:

The docs for frame::runtime, unlike almost all other frame macros, live in the procedural crate, and therefore is ignore-ed code. It would align with our standards better to move them to where it is being re-exported so the code snippets can be live.

[shawntabrizi]

Is this syntax supported?

///   // Pallet with instance.
///   #[runtime::pallet_index(2)]
///   pub type Test2_Instance1 = test2::Pallet<Runtime, Instance1>;

This is what I would expect for Pallet<T, I>

[gupnik]

Is this syntax supported?

///   // Pallet with instance.
///   #[runtime::pallet_index(2)]
///   pub type Test2_Instance1 = test2::Pallet<Runtime, Instance1>;

This is what I would expect for Pallet<T, I>

Yes, this is the way its added now. Will add this in the docify example as well.

[gupnik]

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