[FLEET] Support preserving assets for a previous package version when package is updated

**Describe the feature:**

Add functionality for preserving assets for a previous package version when package is updated.

Proposed solution is to add a `preserve_assets: boolean` field to package manifest and use that as the check for whether or not to delete previous assets on update.

Related to changes in: https://github.com/elastic/kibana/pull/107710 in which support for ML model packages has been added.

The current workflow for model packages requires older versions not be removed when a user updates a package. To prevent previous versions pipeline assets from being removed from ML packages on upgrade, the temporary solution is to skip previous versions pipeline deletion when those pipelines are top-level assets (not within a datastream directory).

This is a temporary solution that works for now because ML model packages are the only packages currently containing top-level pipeline assets so this will not affect other packages.

This issue is to propose a more robust solution.

Start by reviewing the package manifest and the package update workflow, focusing on how previous-version assets are identified and deleted. Compare the proposed preserve_assets field with the temporary handling for top-level pipeline assets; done means package updates preserve assets for versions that opt in without changing deletion behavior for other packages.

Written by the indexing model from the issue text.