`ethexe-rpc`: implement subscription for all promises

[ecol-master]

Problem to Solve

The ethexe-rpc must provide the subscription for all promises produced by ethexe rollup.

Possible Solution

From client side solution must look like this:

/// Client code
let mut subscription = vara_eth_rpc_client.subscribe_promises().await.unwrap();

while let Some(promise) = subscription.next().await {
    some_promise_processing(promise);
}

Notes

No response