Adds example for custom default configs

[gupnik]

Closes #4827

This PR provides an example to create custom default configs for any pallet that could be created externally and shared across runtimes. Such a paradigm can be used in #4056.

It requires the following:

1. A trait that carries those configuration parameters for a pallet that should be shared across runtimes
2. An implementation of that trait registered via register_default_impl
3. Usage of derive_impl at the actual impl site pointing to this default implementation

[bkchr]

Reading the docs, I have no idea how to use this.

[gupnik]

Reading the docs, I have no idea how to use this.

Will update and provide more details.

[franciscoaguirre]

Should we add this to the polkadot-sdk-docs as well? We should probably have a section on macros, even if it is just a list of macros with links to their rust docs

[gupnik]

Reading the docs, I have no idea how to use this.

Updated.

[kianenigma]

How about:

1. One example that demonstrate how this can be used in a pallet, instead of #[pallet::config(with_default)] to create "partial configs" and skip providing defaults for things that we are not sure about, for example to solve the issue of @seadanda.
2. (This is the one I am more interested in) One example of how this can be used in a standalone trait. For example, to build a custom trait like:

trait OpinionatedSystem: frame_system::Config { // more types and fns.}
// register default for this. 

#[frame::pallet] mod pallet {
    #[pallet::config]
    pub trait Config: OpinionatedSystem { .. }
}

mod runtime {
    // Use the default provided
    impl pallet::Config for Runtime { .. }
}

And please focus on making both as self-explanatory and e2e as possible?

[kianenigma]

Should we add this to the polkadot-sdk-docs as well? We should probably have a section on macros, even if it is just a list of macros with links to their rust docs

1. https://paritytech.github.io/polkadot-sdk/master/frame_support/pallet_macros/index.html
2. https://paritytech.github.io/polkadot-sdk/master/polkadot_sdk_frame/pallet_macros/index.html

Although the latter is not working properly, the re-export is not expanded.

[kianenigma]

Reading #4056 (comment) again, the success scenario here is to make a new trait DefaultConfig for system that has a subset of the other types, that allows #4056 to use it.

The manual approach here works, but it does lose all the type safety, FYI. In that, the trait DefaultConfig needs to manually have the same trait bounds.

[kianenigma]

And if we are to lose type-safety, we can alternatively do #5246.