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.

[Baoyuantop]

Thanks for working on this feature. The direction is useful because it turns the #667 workaround into an explicit values.yaml entry.

I do not think this is ready to merge yet. There are two points that should be addressed first:

1. The new schema is added as a top-level proxy_cache, while the actual template and docs use apisix.proxy_cache.zones. That means the schema does not validate the values path users will actually configure. Please either move the schema to the real values path or explain the current schema structure and add validation evidence.

2. This PR changes APISIX startup config rendering, but I do not see a helm template/chart-testing case showing a custom proxy_cache.zones entry rendered correctly. Please add a minimal rendering verification covering name/memory_size and the optional disk/cache_levels fields.

Also, the toYaml -> fromYaml -> len check in the template is not very clear for a list value. Using with or checking the list length directly would be easier to maintain.