Add support for building transactions with multiple signers who aren't the source account

[leighmcculloch]

What

The CLI needs to support for adding a different signer or multiple signers to a transaction without any of those signers being the source account.

Why

The CLI today assumes that the signer of a transaction is the master key of the source account. This is fine for development and for simpler user key setups, but doesn't support more complex signers configurations that the Stellar accounts of organizations (e.g. asset issuers) and security sensitive individuals.

To address those advanced needs the CLI should support constructing transactions that are signed by a different key than the source account, and support being signed by multiple keys.

How

Add a new option --signer that is supported anywhere that --source is supported.

The --signer option can appear zero, one, or many times.

When a --signer is not present, the --source is also the signer.

When a --signer is present one or many, the --source is not a signer, and each signer is looked up separately and signs the transaction in sequence.

Examples

For example, today the CLI supports, and it would continue to support:

soroban contract deploy --wasm ... --source me

And for example, a user should be able to sign the tx with a different key than the source:

soroban contract deploy --wasm ... --source me --signer myotherkey

And for example, a user should also be able to specify multiple keys:

soroban contract deploy --wams ... --source me --signer myotherkey --signer mysecondkey

This change makes the first example in the issue above shorthand for:

soroban contract deploy --wasm ... --source me --signer me

Notes

Note that the use of multiple keys/signers located on a local machine doesn't really increase security, and so the importance of supporting multiple keys is significantly less than the importance of supporting a different key. The critically important feature of this issue is to support a different key for signing. But it's a natural extension to support multiple, and a tiny effort to add it in conjunction.

[tomerweller]

What if the user doesn't want to sign with any key? they just want to go through the build->simulate flow and then take the tx XDR and sign elsewhere?

[leighmcculloch]

I think we should capture that in a separate issue. I'm hoping once #1180 gets broken up into two issues that the issue that comes out of that for skipping signing on any tx built will involve adding a --no-sign or --sign false.

[leighmcculloch]

@tomerweller A separate issue now captures supporting that scenario: #1265