Create subscription to fuel-core TX pool

[ra0x3]

• It's been requested in the past that the indexer allow for read-only access to fuel-core's TX pool at some point in time
• This state/view of the TX pool would be passed to indexer handlers (e.g., same as we do with BlockData)
• Indexer authors would then be able to have read-only access to these transactions for MEV purposes
• This would require adding a GraphQL subscription from Fuel Indexer --> Fuel Core so that Fuel core can pass items that have been added (or removed) from the TX pool to the indexer service
• This would require work with the fuel-core team to understand how best to do this
• This would also require documentation once this functionality has been added
• Lots more fun things can be done here :)

Additional notes

• Since this would be implemented as a GraphQL subscription, we still might need to decide how we're passing the transactions to the handlers
  • For example, will the subscription to Fuel-Core automagically keep track of TX pool items that have been added & removed?
    • Does the indexer have to keep up with any globally mutable state?
• We might have to create a new handle_events (e.g., handle_tx_events) where the serialized Vec<TxPoolItem> is passed
  • Again, this might work very similarly to what we already have
• Performance would be important here in a way that it isn't really for regular block indexers