[epm] child properties of nested objects are not defined in mappings

[klacabane]

Summary

When mapping a nested object in a package's data stream, only the top level object is defined in the mapping and any child properties of that object is discarded. _generateMappings seems to be the function responsible for the translation.

Steps to reproduce the issue

• Set a nested object with at least one property in a data stream (I used the node data stream of the logstash package):

- name: test_nested_fields
  type: nested
  fields:
    - name: first
      type: keyword

• build the package elastic-package build -v
• start an elastic stack elastic-package stack up -v -d
• install the assets of the updated package in Fleet
• inspect the updated data stream mappings, with the above example only the test_nested_fields object is defined without any child properties

[klacabane]

According to these tests this is not the way to define the nested properties. So if we flatten the child property with the dotted syntax the mapping will be correctly created. What's curious is that a mix of nested and flattened properties also works while only nested props don't

# works
- name: test_nested_fields
  type: nested
- name: test_nested_fields.first
  type: keyword

# works
- name: test_nested_fields
  type: nested
  fields:
    - name: second
      type: keyword
- name: test_nested_fields.first
  type: keyword

# does not work
- name: test_nested_fields
  type: nested
  fields:
    - name: first
      type: keyword