[VIRTS-4152] Include exit code in agent's link results

[michael-the-jones]

Description

Dependent on mitre/caldera PR (https://github.com/mitre/caldera/pull/2713).

Include exit code in agent's link results. For the Shellcode, Shells, Native, and Donut executors, exit code will now be returned from the agent. While Shells and Donut exit codes come from the OS/shell itself, Shellcode and Native are hard-coded descriptors used for consistency across executors.

Type of change

• [X] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [X] This change requires a documentation update

How Has This Been Tested?

Tested shellcode, shells, and native code execution paths manually and with assistance of unit tests. Changes are relatively not invasive, and strong code review could provide some level of assurance.

Checklist:

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

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