fixes #8306,8177,8180 - adding incremnetal update api

example api call:

POST /katello/api/v2/content_view_versions/incremental_update

{
"content_view_version_environments": [{"content_view_version_id": 5, "environment_ids": [1,2,3]}],
"add_content": {"errata_ids": ["3068d8b3-5546-4ad7-90ea-8df0c0af5792"]  },
"environment_ids": [1],
"resolve_dependencies": true
}


you may also want to test  'package_ids' & 'puppet_module_ids' as valid content
This question would apply to any of the content types here -- do we care if a user attempts to incrementally update a package/errata/puppet module that exists in a repository they do not have access to?