[Change Proposal] Base packages

There are some settings that could be enabled in most if not all packages, such as conditions, processors or tags.
Some of these settings are managed by Fleet. For example the datasets in input packages. Others need to be included in packages.
An alternative could be to have some kind of base package that includes these common settings. These packages would be referenced by its dependent packages and Fleet would merge both packages.

For an initial MVP it should be possible at least to define common settings and configuration snippets.

We would need to define:
* The spec for these base packages.
* What package types could reference these base packages and how.
* How settings would be merged. Possibly it should be merged with all policy templates.
* How configuration would be merged. We would probably need different base packages for different collection technologies (Beats vs. OTel).