Add agent fields into config file for system tests

[mrodm]

What does this PR do?

Add agent fields into the test system configuration files that would be used to customize Elastic Agents.

Why is it important?

These new settings would allow the developers to specify the required settings (user, pid mode, linux capabilities...) to run the tests of the given Data Stream of that package.

Checklist

• [x] I have added test packages to test/packages that prove my change is effective.
  • Added a new Data Stream to good_v3 test package.
• [x] I have added an entry in spec/changelog.yml.

Related issues

• Depends on https://github.com/elastic/elastic-package/issues/787

[elasticmachine]

:green_heart: Build Succeeded

• Buildkite Build
• Commit: 4f515a7c0b8a120f451f2e860c76375ae155d058

History

• :broken_heart: Build #654 failed e727b914de66b1c8e56bba7d99c0a4a9139b0d82
• :green_heart: Build #648 succeeded 578ad29c0131cc54b3a49389c28c43bb3fdffbd5
• :broken_heart: Build #647 failed 5606364c0254c8a8a49824ee892654034d5a3677
• :broken_heart: Build #646 failed 942b85a4e7e173104d3871fde3bff09e1f24cad9
• :broken_heart: Build #644 failed ef453274128954a8385e5d2a9dd6cc4d6e9dae04
• :broken_heart: Build #643 failed 85c61be2b63bcd11904351142ba33bdee1c38dd7

cc @mrodm