[Fleet] install Entity definitions included in packages

[klacabane]

Summary

We'd like to include Entity definitions in packages. When included in a package, entity definitions should be loaded by the fleet plugin on package installation and further logic should be applied to install them.

In their current state (Entity definitions are tech preview), installing a definition means installing ingest pipelines, index templates and transforms. The logic to do that is hosted in the entity_manager kibana plugin and exposed by a client that can be consumed by fleet.

This issue tracks feasibility and discussion around the implementation

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[klacabane]

I have a draft fleet implementation #193416 relying on elastic/package-spec#804.

While the package-spec defines definitions under <package>/kibana/entity_definitions, they're not stored as saved objects like sibling assets but instead as a json blob. As such I've not included definitions as a KibanaAssets type but as a separate step in the installation process. Happy to reconsider this if it does not make sense.

The current implementation #193416 is fairly straightforward, it will install/update definitions when appropriate and also delete them. I'd love to get an initial feedback on this to understand if this would be the right direction, what is missing and what else should be considered.

[klacabane]

I've closed #193416 as we'll look into another approach to include the definitions for now, I'll link a new PR soon