feat: txn command to simulate

[willemneal]

Part of #1265
Adds

• txn simulate - simulate and assemble transaction

Future PR:

• signer::Stellar trait and a default InMemory implementation needed for txn sign
• txn sign - sign a given transaction
• txn inspect
• txn send - sends a signed transaction to the PRC (not useful without sign)

Left

• Tests

[leighmcculloch]

I pushed some commits getting main merged in.

This PR looks like it's still a little ways away from being ready. Left some inline comments.

Would be a good idea to add tests for these commands too, from the outside-in.

[leighmcculloch]

Given the sensitivity of the sign command, I'd like to shift that work into a separate PR, so that we can get the other commands merged sooner, then follow up and take our time with the sign command. There should be litle to no need for the commands to be added together, so I think that'll be fine, but please lmk @willemneal if you think that'd be a problem. cc @janewang

[willemneal]

@leighmcculloch I've removed send because it has hard to test without a proper sign command. So there is another PR to add all the remaining besides inspect.

[chadoh]

Nice clean code! I really like the tests. I have some questions, before I approve.

[chadoh]

Looks great, thanks!