Move cluster agent frontend to Core

What does this MR do and why?

Part of making Cluster Agents available in Core (&6290 (closed)).

These changes will not yet be visible in Core, as there is still a helper method preventing the link to it being rendered:

  # app/helpers/clusters_helper.rb
  def display_cluster_agents?(_clusterable)
    false
  end

  # ee/app/helpers/ee/clusters_helper.rb
  override :display_cluster_agents?
  def display_cluster_agents?(clusterable)
    clusterable.is_a?(Project) && clusterable.feature_available?(:cluster_agents)
  end

We leave this unchanged until everything has been moved, at which point it will serve as a simple way to enable the feature.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

#340355 (closed)

spec/frontend/clusters/agents/components/show_spec.js

 import { GlAlert, GlKeysetPagination, GlLoadingIcon, GlSprintf, GlTab } from '@gitlab/ui';
 import { createLocalVue, shallowMount } from '@vue/test-utils';
 import VueApollo from 'vue-apollo';
-import ClusterAgentShow from 'ee/clusters/agents/components/show.vue';
-import TokenTable from 'ee/clusters/agents/components/token_table.vue';
-import getAgentQuery from 'ee/clusters/agents/graphql/queries/get_cluster_agent.query.graphql';
+import ClusterAgentShow from '~/clusters/agents/components/show.vue';
+import TokenTable from '~/clusters/agents/components/token_table.vue';
+import getAgentQuery from '~/clusters/agents/graphql/queries/get_cluster_agent.query.graphql';
 import { useFakeDate } from 'helpers/fake_date';
 import createMockApollo from 'helpers/mock_apollo_helper';
 import waitForPromises from 'helpers/wait_for_promises';