feat: create an append_message_output method

[iqdecay]

Closes #579 by creating an append_message_output method that works similarily
to append_variable_output.

[iqdecay]

@mohammadfawaz I'm not sure I understood the context for this so I didn't include Sway tests as I would usually do, do you have some Sway code to show me where this API could interact?

[nfurfaro]

@iqdecay 2 points:
1.) My draft PR is just waiting for this PR to land and be released so I can test it correctly!
Similar to how the tro(transfer to output) opcode requires that a variable output be appended in order to work, the smo opcode (send message output) requires a message output be appended to the call in order to work.

2.) we should consider if there's any other way to approach testing, as we've already seen that there's a circular dep issue between the stdlib (the tests use the SDK) and the SDK (which has some tests which use sway (and the stdlib).
It might be that we work on features like this one in tandem, and you can run your tests pointing at a stdlib branch other than master where the supporting changes have been made already. Once the SDK changes are released, the stdlib changes can merge to master, and then the SDK tests can be updated to use the master stdlib branch...

Open to ideas here, but the above approach would (imo) be better than the alternative, which is temporarily disabling tests and related functionality in the SDK during this process of re-syncing dependencies.

[hal3e]

Good work! I left a nit and a question..