Abstract api-client main features behind a trait

[clangenb]

This would greatly simplify the separation between the core logic and the custom business logic in the integritee-worker in case the node has some customizations, which are incompatible with the default api-client type, e.g., the signed extra if pallet_asset_tx_payment is used.

Excerpt from the envisioned trait

pub trait Api {
          fn get_storage<V>() -> Result<Option<V>>;
     
          fn subscribe_events();

        // etc.
}