Description

This PR does the following:

• removes all committed files coming from a third party repository (namely capnproto) and adds a submodule instead
• adds the submodule initialization step in the makefile and the various GH action jobs

Motivation and Context

I am not entirely sure why all the external libraries have been committed in the VTR codebase, but I think that it should be much easier to handle third party projects with submodules, so that, in case we need to update them, we can just bump the submodule instead of committing the changes that do happen upstream.

How Has This Been Tested?

regression tests

Types of changes

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed