Make transaction approval screen aware of pending transactions

[onyb]

Community posts:

• https://community.brave.com/t/i-cant-send-token-or-make-a-swap
• https://community.brave.com/t/send-error-from-wallet-to-all-different-addresses/443121

The transaction approval screen must be aware of pending transactions stuck in the mempool, and be able to display recommendations to the user based on the nonce values of those pending transactions. For example, one recommendation could be to adjust the gas settings in a manner that evicts the pending transactions with the same nonce values.

There seems to be an RPC method that we can use for this: web3.eth.getPendingTransactions([, callback])

Not to be confused with unapproved "pending" transactions in Brave Wallet.

[darkdh]

Or we can do eth_getTransactionCount with pending as we currently use latest in nonce tracker.

"latest" - String: The latest block (current head of the blockchain)
"pending" - String: The currently mined block (including pending transactions)

And our nonce tracker now only considers local pending transactions(submitted) so the issue can easily happen when sending transactions which will get stuck in mempool between different devices.

With eth_getTransactionCount + pending, there will only be core changes because core will make sure nonce field is properly filled before signing and publishing so it won't get error of conflicting with nonce of pending transactions.