[Change Proposal] Support additional asset types in content packages

[kpollich]

Follow-up for #351

Today, content packages only supports dashboards or simple Kibana saved objects that can be directly imported. We should expand this support to allow for all Kibana asset types listed here: https://github.com/elastic/package-spec?tab=readme-ov-file#supported-assets

[kpollich]

@jsoriano Now that content packages are GA, it'd be great to provide a path to get the prebuilt rules package moved over to type: content. I think we need to add security rules as a supported asset type and then things should work pretty much as expected? Not sure if I'm missing any complexity there. I'm going to schedule this for our next sprint to take a look.

[jsoriano]

Yes, in principle we can start adding additional assets. For prebuilt rules I think we also needed to improve support for big files and packages in Fleet.