Add CAP_CHOWN capability if package requires root privileges

[mrodm]

By default, elastic-package drops all capabilities for the Elastic Agent container when running system tests. Just adds the capabilities added explicitly in the system test configuration file.

Starting with 8.17.1-SNAPSHOT, packages that require root privileges cannot start the Elastic Agent container and they are failing with this error:

Error: the command is executed as root but the program files are not owned by the root user. execute the command as the user that owns the program files

To avoid this error with packages that require root privileges when starting the Elastic Agent container, it is required to add the CAP_CHOWN capability if it was not added by the package previously.

Example of packages failing using 8.18.0-SNAPSHOT:

• https://buildkite.com/elastic/integrations/builds/20201#019448b8-8404-4ea1-a8d6-3ec5ebd1244d

This root privileges could be defined as part of the manifest:

agent:
  privileges:
    root: true

or in each system test configuration file (e.g. example):

agent:
  user: root