Custom default configs using derive_impl

[gupnik]

As @kianenigma suggested in this comment, we should be able to provide custom default configs and use derive_impl to share those across runtimes.

The following could be possible way:

pub trait CustomDefaultConfigProvider {
	type RuntimeOrigin;
	type RuntimeCall;
	type RuntimeTask;
}

pub struct CustomDefaultConfig;

#[crate::register_default_impl(CustomDefaultConfig)]
impl CustomDefaultConfigProvider for CustomDefaultConfig {
	#[inject_runtime_type]
	type RuntimeOrigin = ();
	#[inject_runtime_type]
	type RuntimeCall = ();
	#[inject_runtime_type]
	type RuntimeTask = ();
}

struct Runtime;

#[derive_impl(CustomDefaultConfig, no_aggregated_types)
impl Config for Runtime {}

If this works, we should add some documentation (ideally using docify) on how to use it.