Allow specifying default cluster pool assignments in admin config

[katrogan]

Tracking issue

N/A

Describe your changes

Allows specifying cluster pools in helm chart. Change added in flyteadmin here: 07ff730

Tested by setting

clusterpool_config:
  clusterPools:
    clusterPoolAssignments:
      development:
        pool: pool1

in values.yaml and asserting that generated helm chart changes rendered:

+  clusterpool_config.yaml: | 
+    clusterPools:
+      clusterPoolAssignments:
+        development:
+          pool: pool1

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

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