Add support for pending block ID in starknet_subscribeTransactionStatus

[sistemd]

Right now we return an error when blockId: pending, but even with blockId: latest we do send pending updates for the current block. Instead blockId: latest should not return pending updates for the current block (i.e. current block at time of subscription), whereas pending should. I think this is what needs to change in the code:

• This code should not run when block ID is pending.
• If block ID is latest, we need to fetch the current block number n when starting the subscription, and add a special case so that this and this code only runs when the block number is larger than n+1 (note: this is only for the case where block ID is latest!). This ensures that a pending update for the upcoming block is not sent in the case where block ID is latest.