feat: Allow proxy_cache zones to be configured from values.yaml

[brmejia]

This PR add missing functionality reported here #667

Additional proxy_cache zones can be defined using values.yaml like this:

apisix:
  proxy_cache:
    zones:
      - name: memory_cache
        memory_size: 256m

[brmejia]

Could someone give feedback or review this PR?
Thanks

[oksman]

This would be a very nice feature! Currently it seems we need to overwrite the whole config.yaml which isn't that nice.