[Idea] Add validation check against ECS fields

At the moment, a package can contain any fields. One important thing about ECS is that it can be extended with any fields but fields should not conflict with ECS, for example `host` vs `host.name` (keyword vs object). On the registry side we validate packages and it would also be possible to validate that all the fields defined in the `fields.yml` are not conflicting with ECS.

This is mainly to document the idea for now.