add the ability to declare multiple impl blocks

[pkhry]

Description

Adds the ability to split single impl_runtime_apis into multiple modules through sp_api::impl_runtime_apis_ext.

Example block would look like:

impl_runtime_apis! {
    impl trait Example<Block> for Runtime {
             fn example() {}
    }
    // `use` makes metadata from and runtime_versions visible to the main impl block.
    use other::*;
}

#[sp_api::impl_runtime_apis_ext]
mod other {
        use super::*; 
     	impl super::Example2<Block> for Runtime {
		fn same_name() -> String {
			"example".to_string()
		}
	}
}

also see: #3067

Review Notes

The main difference between impl_runtime_apis and impl_runtime_apis_ext is the fact that the latter doesn't generate base runtime structures and doesn't track the imports to aggregate them inside the metadata and runtime_api_versions.

TODOs:

• RUNTIME_API_VERSION
  was a const before, i've moved it to a standalone function inside the main impl block.
  The question is should it be tied to the runtime via trait e.g RuntimeApiVersion and if yes where should it go?(sp_api or sp_version)
  I've tried to figure out a way to use a const but given that all of the const declarations contain data inside a std::borrow::Cow and and there's no way to get value out in const expression and i've failed to do so, if someone has any idea how to get around this i would really appreciate some input.

  		pub fn runtime_api_versions() -> #c::ApisVec {
  		let api = #c::vec::Vec::from([RUNTIME_API_VERSIONS.into_owned(), #(#modules::RUNTIME_API_VERSIONS.into_owned()),*]).concat();
  		api.into()
  	}

[pkhry]

/cmd fmt

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