[discuss] Limited packages & multiple policy templates

Hi, last year we introduced a field, `policy_templates.[0].multiple: boolean`, that lets [Kibana know](https://github.com/elastic/kibana/pull/70542) whether a package can be configured for an agent policy more than once or not: https://github.com/elastic/package-registry/issues/350

This flag is currently in effect for `endpoint` (that is, Endpoint package can only be added **once** to an agent policy) and, AFAIK, no others. Now that packages can have multiple `policy_templates` (https://github.com/elastic/package-spec/pull/137), I'd like to discuss whether this flag still makes sense at the policy template level, or if we should consider moving it to the package level.

My preference is to move it to the package level, semantically that makes more sense to me with the original intention of this field, but I realize that this might change now with packages exporting multiple integrations. Do we foresee a use case where we will want to apply this restriction at the policy template/integration level?