Define CommandMessage pattern and implement first messages

[TimoPtr]

Summary

Come up with a pattern to define with a strong type the Commands that we send to the frontend that uses the command JSON discriminator. Since a sealed class member cannot have twice the same discriminator (otherwise it cannot de-serialize properly), I had to come up with this pattern where we override the invoke operator of an object to keep a nice API.

Checklist

• New or updated tests have been added to cover the changes following the testing guidelines.
• The code follows the project's code style and best_practices.
• The changes have been thoroughly tested, and edge cases have been considered.
• Changes are backward compatible whenever feasible. Any breaking changes are documented in the changelog for users and/or in the code for developers depending on the relevance.

Any other notes

I don't like it but I didn't find any other way for the Payload to be a JsonElement, using a template breaks the serialization engine.