[Change Proposal] Ingest pipelines support in content packages

Even if content packages are not thought to configure how data is ingested, there are some scenarios where ingest pipelines may be useful:
* Adapting data for other components included in content packages, what can provide some flexibility when supporting different versions or types of collectors.
* Transforms based on incoming data (not supported yet in content packages).

To properly support ingest pipelines, we would also need to:
* Ensure that auto-discovered content packages can attach their pipelines to the matching indexes or data streams. In content packages that include multiple pipelines we may need a new setting to indicate what pipeline to use when auto-discovered.
* Support mappings definition in content packages, so fields created by pipelines have proper mappings.
* Make `elastic-package test pipelines` work with content packages.