Add `container_policy` to policies controller

What does this MR do?

As discussed in !66543 (comment 641272656), this MR is a follow-on to allow the policies edit page to support container runtime policies, in addition to the existing scan execution policies. We intend to eventually move the container runtime policies to be managed by a security policy project, which will allow them to be retrieved by Security::SecurityOrchestrationPolicies::FetchPolicyService, but we currently do not have the engineering capacity to do so. In the interim, we will use NetworkPolicies::FindResourceService to retrieve the policy when the policy type is container_policy. This duplicates some of the logic in Projects::ThreatMonitoringController, but this controller will be deleted at the completion of this feature.

Screenshots or Screencasts (strongly suggested)

N/A - Still needs frontend

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

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