[VIRTS-4153] ]Update server to account for exit_code from agent

[michael-the-jones]

Description

Allows agents to send the "exit_code" field with results to the server. The exit code is included in reports.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

Tested with updated Sandcat executors from the related PR (mitre/sandcat#427).

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• [Adding PR in mitre/fieldmanual] I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

[github-actions]

This issue is stale because it has been open 20 days with no activity. Remove stale label or comment or this will be closed in 5 days

[elegantmoose]

Single failing test unrelated to PR.