fix: ensure TxFactory has acc & seq fields when simulating tx in generate-only mode

[clevinson]

Description

On regen ledger when using the CLI I noticed that --gas auto does not work in --generate-only mode. I was able to reproduce it on Cosmos-SDK master with bank send msg's as well:

simd tx bank send $ADDR1 $ADDR2 10stake --generate-only --gas auto

Failed with the error:

Error: rpc error: code = InvalidArgument desc = account sequence mismatch, expected 2, got 0: incorrect account sequence: invalid request

After digging around on the SDK, I found that when broadcasting a tx we often call txf.Prepare() on the TxFactory to add acc number & sequence in case it isn't already set explicitly via CLI flags. However, we don't do this when calling txf.PrintUnsignedTx(). Usually this isn't a problem, but acc numbers & sequences are actually required when simulating a tx for calculating gas (e.g. if --gas auto is set).

This PR fixes the above issue by running txf.Prepare() prior to simulating the transaction for gas calculation in --generate-only mode.

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[tac0turtle]

utACK

should we backport to 0.45?