Expose library/dependency metadata within the rego struct

[mrueg]

Currently konstraint only exposes the metadata of the object it parses. For libraries the rego policy imports, it removes all metadata.

It would be nice if there is a way to expose the metadata from the import as well.

We intend to use it the following way:

Policy Foo

# METADATA
# title: Foo
# description: Only applies to Deployments in the 'foo', 'bar', and 'baz' namespaces.
# custom:
#   matchers:
#     kinds:
#     - apiGroups:
#       - apps
#       kinds:
#       - Deployment
#     namespaces:
#     - foo
#     - bar
#     - baz
package main

import future.keywords.if
import future.keywords.contains
import data.lib.core
import data.exemptions.foo

violation contains msg if {
    not is_exempt
    msg := sprintf("%v is a Deployment in the foo, bar, or baz namespace", [core.resource])
}

with a cluster specific exemption in the exemption library

# METADATA
# custom:
#  matchers:
#    namespaces:
#    - additional_namespace_we_want_to_restrict

import data.lib.core

is_exempt {
...
}

The default templating doesn't need to make use of this, but if exposed, the custom templating could use this to include the namespace matcher as well easily.