Callback constructor for FutureLink

[kellpossible]

Description

This is an initial implementation, just requesting comments at this stage. I still need to implement some examples.

• Add a new method FutureLink::callback_future(), and implement it for ComponentLink and AgentLink.
• Refactor the FutureLink::send_future() to take Into<Message> instead of Message to make it match ComponentLink::send_message() more closely, and more convenient to use.

Implements #1271

Checklist:

• I have ran ./ci/run_stable_checks.sh
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works

[kellpossible]

I'll also update the documentation for ComponentLink::callback to reference this feature (if the PR for the docs on the other ComponentLink methods gets approved).

[jstarry]

@kellpossible these changes look solid to me!

[jstarry]

@kellpossible cool if I merge this?

[kellpossible]

I never added an example, do you think it's okay?

[jstarry]

Yeah, that's fine with me

[kellpossible]

It also lacks tests. But if its good to merge then go ahead. I also ran the tests I believe but didn't check the box because I hadn't made the example.

[kellpossible]

Great, thanks @jstarry !