Fix signing with a secure store key

[elizabethengelman]

What

• Fixes Fix signing with a secure store key with stellar tx new ... #2006
• which is a sub-issue of Signing txs used with a key that was created with --secure-store is broken #1980

Todo:

• make sure that stellar tx new set-options --set-revocable --source mykey2 works (i.e. we should be able to sign newly created tx with a secure store key)
• make sure that tx sign supports secure store

Why

This was previously broken. The current workaround is to use --build-only and then pipe that output into stellar tx sign which does work. This works because stellar tx sign uses a different code path (config::sign_with::Args.sign_tx_env) that gets the signer from the Secret, instead of trying to get the private key directly.

Known limitations

• There are a couple of subtasks to handle the rest of the work required for the main issue Signing txs used with a key that was created with --secure-store is broken #1980 to be complete:
  • refactoring signing so there is just one code path: Refactor all signing to go through the same code path #2008
  • contract invoke with secure store keys is still broken because sign_soroban_authorizations expects a SigningKey argument: Fix signing with a secure store key for stellar contract invoke #2007