feat: txn commands to sign, simulate, and send txns

[willemneal]

Part of https://github.com/stellar/soroban-cli/issues/1265 Adds

• txn send - sends a signed transaction to the PRC
• txn sign - sign a given transaction
• txn simulate - simulate and assemble transaction
• signer::Stellar trait and a default InMemory implementation needed for txn sign

Left

• [ ] Tests

[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]

Friend bot seems to be failing.

[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.