QGB relayer wait for transactions to be confirmed

[rach-id]

In #354, we added a simple mechanism for the QGB MVP to wait for transaction confirmation.
However, we will need to have a better mechanism for QGB 1.0.

Proposition

We can have a generic method that takes a transaction hash and a number of confirmations. Then, it subscribes to an EVM endpoint and keeps listening for new blocks.
Once a block is received, it checks if the transaction is included or not.
Then, it waits for a certain number of confirmations before returning.
This would allow the QGB to send the right transactions and not send a lot of them for the same valset which will be eventually rejected.