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

example api call:

POST /katello/api/v2/content_view_versions/incremental_update

{
"content_view_version_ids": [26],
"add_content": {"errata_ids": ["3068d8b3-5546-4ad7-90ea-8df0c0af5792"]  },
"environment_ids": [1],
"resolve_dependencies": true
}
Could we call this `:content` instead?  Are we ever going to remove content?