Add cache layer to ApiService to reduce duplicated RPC queries

[AndreiEres]

We usually run parachains-tracker to follow multiple parachains and query rpc for information related not only to the particular parachain, e.g. validator groups.

async fn on_inherent_data() {
	let validator_groups = self.executor.get_backing_groups(self.node_rpc_url.as_str(), block_hash).await?;

We can reduce a huge amount of rpc queries, adding a cache layer to api service.