Add support for recursive structures in packetbeat's ECS field handlers

[fearful-symmetry]

Proposed commit message

fixes #42108

This fixes the behavior of MarshalMapStr() so that it'll properly recurse through nested structs when turning the ecs.Fields object into a map. This might technically be a breaking change, since the process.parent field will
now produce a different (but correct) map.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/sec-linux-platform (Team:Security-Linux Platform)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @fearful-symmetry? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[nicholasberlin]

LTGM if CI agrees.

[andrewkroh]

LGTM. Fixes the bug at hand.