Unify the subscribe pending transactions in `eth_subscribe` method

[fenghaojiang]

Rationale

Why should this feature exist?

I notice that there are differences from go-ethereum and erigon to use eth_subscribe to get full body of transactions.
In go-ethereum:

c.EthSubscribe(ctx, ch, "newPendingTransactions", true) 

In erigon:

newPendingTransactionsWithBody

I want to make a PR that make two clients compatible and would keep the newPendingTransactionsWithBody in erigon.

What are the use-cases?
Users do not have to care about which client the RPC provider use and just use the same method of subscription.

Implementation

Do you have ideas regarding the implementation of this feature?

Yes.

Are you willing to implement this feature?

Yes.