[Change Proposal] Support ML modules in content packages

[tommyers-elastic]

ml_module asset type is currently supported in integration packages, but not content packages; this proposal suggests we support it there too.

https://github.com/elastic/package-spec/blob/main/spec/integration/kibana/spec.yml#L86

[kpollich]

@tommyers-elastic What's the priority level on this? We're in the process of cutting a 3.6.1 for package-spec right now with some key dependencies for composable packages and OTel input packages and these are our top priorities.

Happy to help support getting this in soon, but we'll want to do a follow up 3.6.2 release next week sometime.