How to set fields on Transactions

[leighmcculloch]

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We don't have to do this in this PR, but we should make sequence number an option, that if isn't set uses the nxt seq num lookup, otherwise uses the value passed in. A good follow up to add later.

Originally posted by @leighmcculloch in https://github.com/stellar/stellar-cli/pull/1551#discussion_r1783846862

cc @willemneal