Allow to override ECS fields type from keyword to constant_keyword

[jsoriano]

Support the case of defining the value of a constant_keyword for fields that are defined as keyword in ECS.

Current workaround is to define the whole field on each data stream instead of importing it, so the constant_keyword type can be set along with a value. Something like this:

- name: event.dataset
  type: constant_keyword
  description: Event dataset
  value: apache.access

With this change, it will be allowed to reuse the definition from ECS:

- name: event.dataset
  external: ecs
  type: constant_keyword
  value: apache.access

This is helpful to have consistent descriptions and other settings in more complex fields.

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-09-12T08:48:37.998+0000

• Duration: 37 min 1 sec

Test stats 🧪

Test	Results
Failed	0
Passed	829
Skipped	0
Total	829

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (34/34)	💚
Files	66.929% (85/127)	👍
Classes	61.667% (111/180)	👍
Methods	48.834% (356/729)	👍
Lines	32.231% (3225/10006)	👍 0.027
Conditionals	100.0% (0/0)	💚

[mtojek]

Thanks for leaving a comment there :) You can get lost with all these rules.