Publish messages crate on crates.io

[Patrik-Stas]

Objective

• Start publishing the current messages on crates.io

Rationale

• Enable composability in rust ecosystem, this can bring more contributors to messages crate itself
• Drive us to improve APIs and module visibility within the crate
• Develop flow to publish crates and go forward with focus on modularity - for example, we could follow up this task by publishing also diddoc, ddo-resolver, protocols, etc.

Open questions:

• How to name the crate?
  • This begs to think a bit forward, how would we publish all the other crates as well? I'd imagine aries-vcx-<foo> could work fine, with special exception aries-vcx crate keeping it's current name. aries-vcx. This would bring us to aries-vcx-messages for messages crate.
• How to version the crate?
  • Perhaps we can start with universal versioning as we currently do - all crates in the repo being increment at once upon every release. Far from perfect, but might be a good starting point. We would keep versioning 0.x.x and once we would "graduate' some crate to be version on its own, we could start from 1.x.x