[Change Proposal] Support ML trained models in integration packages

Please read the section on [Change Proposals in the Contributing Guide](https://github.com/elastic/package-spec/blob/master/CONTRIBUTING.md#change-proposals) and flesh out this issue accordingly. Thank you!

#### What problem the proposal is solving.

Users should be able to easily install supervised ML models - ML Domain Generated Algorithm detection model and Problem child - and related assets. This requires creating packages for each of these supervised models. Integrations will need to support the installation of these models.

#### Where the solution will need to be implemented, i.e. which parts, if any, of the Elastic Stack will be impacted.

- [x] Create PR in the https://github.com/elastic/package-spec repo to define the ml_model asset type
- [x] PR - https://github.com/elastic/package-spec/pull/204
- [x] Create PR in https://github.com/elastic/kibana to add support (installing the asset using the existing create trained model API) for the model asset type into Fleet
- [x] PR - https://github.com/elastic/kibana/pull/107710
- [ ] Create PR in https://github.com/elastic/integrations to add the model JSON containing the model and other model package assets themselves

- After testing the module, promote to the package to the package registry service for first the snapshot environment, then staging, and finally the production environment.