adds milestone request to message protocol

[acha-bill]

Description

Adds MS_REQUEST to message protocol.

Fixes #1793

Type of change

• Enhancement (a non-breaking change which adds functionality)

How Has This Been Tested?

• Existing unit tests pass
• Added new tests

Checklist:

• [x] My code follows the style guidelines for this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes