Function fetch_authorities

pub fn fetch_authorities<A, B, C>(
    client: &C,
    parent_hash: B::Hash,
) -> Result<Vec<A>, Error>
where
    A: Codec + Debug,
    B: BlockT,
    C: ProvideRuntimeApi<B>,
    C::Api: AuraApi<B, A>,

Load the current set of authorities from a runtime at a specific block.