Add support for synthetic_source_keep = none

[mjwolf]

Add support for synthetic_source_keep mapping in generated elasticsearch component files, and add this mapping field to all ECS fields that represent sets.

synthetic_source_keep = none indicates that field is an unordered set, and helps improve storage efficiency with Elasticsearch logsdb index mode.

• Have you signed the contributor license agreement? y
• Have you followed the contributor guidelines? y
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process? n/a
• If submitting code/script changes, have you verified all tests pass locally using make test? y
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? y
• Is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed. y
• Have you added an entry to the CHANGELOG.next.md? y

[andrewkroh]

LGTM