Improve pod logs API

What does this MR do?

• environment.pod_names uses reactive caching. Change the Pod Logs API to return 202 accepted when environment.pod_names is still being calculated.
• Call environment.pod_names in the PodLogsService instead of the EnvironmentsController so that it is easier to test and we can get a pod_name from the returned list if needed.
• Return pod_name, container_name from ee/app/models/ee/clusters/platforms/kubernetes.rb to the PodLogsService which will return them to the EnvironmentsController, which in turn will return them in the response to indicate which pod and container the logs are from.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team