Add SEP0029 memo required check.

[abuiles]

This PR extends server.submitTransaction to always run a memo required check before sending the transaction. Users can opt-out by setting skipMemoRequiredCheck to true:

server.submitTransaction(tx, {skipMemoRequiredCheck: true})

The check is only run if any of the operations included in transaction is of type

• payment
• pathPaymentStrictReceive
• pathPaymentStrictSend
• mergeAccount

If the transaction includes a memo, then memo required checked is skipped.

See SEP0029 for more information.