[Change Proposal] Support Torch `.pt` ML trained models in integration packages

## What problem is this proposal solving?

Users should be able to easily install supervised ML models. Current, integrations support DataFrame Analytics models, as introduced in
* https://github.com/elastic/package-spec/issues/216

Now we have more [supported language models](https://www.elastic.co/guide/en/machine-learning/current/ml-nlp-model-ref.html). They are shipped in a `.pt` format instead of `.json` configurations. Integrations will need to support the installation of these models.

* Note: Trained model API [compressed_definition](https://www.elastic.co/guide/en/elasticsearch/reference/current/put-trained-models.html#ml-put-trained-models-request-body). The .pt models will need to be converted to base64 format to place into the integration packages.
* Note: To support NLP models, support for the add trained models' vocabulary is required as well. [link](https://www.elastic.co/guide/en/elasticsearch/reference/current/put-trained-model-vocabulary.html)

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

- [ ] Create PR in the https://github.com/elastic/package-spec repo to update the ml_model asset type. Possibly in [this file?](https://github.com/elastic/package-spec/blob/main/spec/integration/elasticsearch/spec.yml)
- [ ] PR -
- [ ] 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. Possibly the `handleMlModelInstall` [chunk](https://github.com/alvarezmelissa87/kibana/blob/b83e6c61506ceba55cf932205516eb725ab09dd0/x-pack/plugins/fleet/server/services/epm/elasticsearch/ml_model/install.ts#L45)
- [ ] PR -
- [ ] ? Create PR in https://github.com/elastic/integrations to add the model .pt containing the model and other model package assets themselves

## Additional information

Currently, models are constrained to 400mb size. We've yet to require an increase, but it's worth noting in the future if we do develop larger models.

Links:
* Eland .pt: https://github.com/elastic/eland/blob/79d9a6ae29624d15840a48465fc2aa87dcbf7ace/eland/ml/pytorch/traceable_model.py#L62
* Trained model API [compressed_definition](https://www.elastic.co/guide/en/elasticsearch/reference/current/put-trained-models.html#ml-put-trained-models-request-body). The .pt models will need to be converted to base64 format to place into the integration packages.