Proto: Inject an internal message when migrating to proto_alpha

Context

This MR introduces a new kind of internal message, intended to be injected by the economic protocol when a protocol migration happens.

The purpose is twofold: allow kernels to know a protocol upgrade happened, and allow the PVM to upgrade itself to new functionalities when it happens.

This MR only deals with purpose 1, that is the injection of the message, that the kernel can interpret as it sees fit.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR