Implementation for newPendingTransactions (eth_subscribe)

[stana-miric]

Description

This PR introduces implementation of newPendingTransaction as a parameter of eth_subscribe. User can subscribe to this parameter and receive updates when the new tx is added to the tx pool.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

Custer is run using e2e test which periodically inserts transactions

 func TestE2E_NewTx(t *testing.T) {
	
        sender, err := wallet.GenerateKey()
	require.NoError(t, err)

	cluster := framework.NewTestCluster(t, 4,
		framework.WithNativeTokenConfig(fmt.Sprintf(nativeTokenMintableTestCfg, sender.Address())),
		framework.WithPremine(types.Address(sender.Address())),
	)
	defer cluster.Stop()

	cluster.WaitForReady(t)

	acct, err := wallet.GenerateKey()
	require.NoError(t, err)

	toAddr := acct.Address()

	srv := cluster.Servers[0]
	t.Log("**JSON RPC ADDR", srv.JSONRPCAddr())

	for i := 0; i < 100; i++ {
		txn := cluster.SendTxn(t, sender, &ethgo.Transaction{
			To:       &toAddr,
			Value:    big.NewInt(1),
			GasPrice: 0,
		})
		require.NoError(t, txn.Wait())
		require.True(t, txn.Succeed())
		t.Log("Tx", i, "- hash:", txn.Receipt().TransactionHash.String())
		time.Sleep(5 * time.Second)
	}
}

In a separate console we subscribed to newPendingTransaction events using following command:
wscat -c ws://localhost:12001/ws -x '{"jsonrpc":"2.0", "id": 1, "method":"eth_subscribe", "params": ["newPendingTransactions"]}' -w 120

Documentation update

@DannyS03 if we have documented which json rpc calls we support, then we should add newPendingTransaction as a part of eth_subscribe

[Stefan-Ethernal]

Generally looks good, leaving some feedback to consider.

[vcastellm]

Late LGTM