Add revision support for transaction entries in client

[grossvater]

What would you like to be added or enhanced When iterating over entries in a transaction, revision should be available for each entry, e.g:

txs, err := client.TxScan(ctx, &schema.TxScanRequest{
		InitialTx: next,
                ...
})

txl := txs.GetTxs()
for _, tx := range txl {
    for _, e := range tx.Entries {
        revision := e.GetRevision()  // missing
    }
}

Why is this needed Revision is an important piece of information when iterating transactions, and today for getting it, another call to History() is needed, then scanning the result looking for the transaction of interest in order to find the revision.

Additional context See also this discussion (in the context of an embedded store, but still, the same need).