ECS fields can include normalization rules, indicated by the normalize parameter.

By now only the array rule exist, that indicates that values should be included in the
field as an array, even if there is a single element.

Validate this on test results for packages with format_version >= 2.0.0.

Things to check in this review:

• This new validation is only enabled for packages using package spec v2. This is the first time we do something like that in elastic-package.
• Field validators can be configured now with the spec version of the package, this is done for all test runners that validate fields.
• Apache test package has been upgraded to version 2.0.0 of the spec. This illustrates the kind of changes that package developers will need to do when updating to this major. In this case it was:
  • Modify the pipelines to change event.category and event.type to arrays as for this PR.
  • Fix the license fields as for Add field for source license and condition for subscription package-spec#355.
• ECS has been upgraded in the Apache test package to a version that has normalization rules.

Part of #615 and elastic/package-spec#399.