[Change Proposal] Relax validations on required fields

We currently check at the spec level that some fields are required, here: https://github.com/elastic/package-spec/blob/5325b7e43d4b49cd6fdb90aa24705095911211e6/code/go/internal/validator/semantic/validate_required_fields.go#L14-L25

The definition of these fields is already provided in many cases by `ecs@mappings`, and its definition doesn't grant that they are correctly used, this can be only checked with system tests. Current implementation forces packages developers to include this boilerplate fields.

So maybe we should remove the expectation on these fields from the spec, and ensure that `elastic-package` can detect misuses or absence of these fields.