Revert some flattened fields to object type

[axw]

Motivation/summary

Revert some fields from flattened back to object to align with how we will dynamically map those fields in the future. We keep a few APM-specific fields that may cause mapping conflicts as flattened for now.

Object-type fields:

• http.request.env.*
• http.request.cookies.*
• http.request.headers.*
• http.response.headers.*
• span.message.headers.*
• transaction.message.headers.*

Flattened-type fields:

• error.custom (APM-specific arbitrary object structure)
• error.exception.attributes (APM-specific arbitrary object structure)
• error.exception.stacktrace (APM-specific structured stack trace)
• error.log.stacktrace (APM-specific structured stack trace)
• http.request.body (may be either a string or object, and object fields are arbitrary so therefore may cause conflicts)
• span.stacktrace (APM-specific structured stack trace)
• transaction.custom (APM-specific arbitrary object structure)

Checklist

- [ ] Update CHANGELOG.asciidoc
- [ ] Update package changelog.yml (only if changes to apmpackage have been made) (reverts an unreleased change made in #12102)
- [ ] Documentation has been updated

How to test these changes

• Check that error/exception stack traces show up correctly in APM UI
• Check that HTTP request/response headers show up in APM UI

Related issues

#11528
#11529

[mergify]

This pull request does not have a backport label. Could you fix it @axw? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.