feat(dcgw): Custom plugin config with managed cache

[cloudjumpercat]

Description

Fixes kapa issue

Content source: https://kongstrong.slack.com/archives/C070FKK6GBT/p1777967344486189?thread_ts=1777649727.164449&cid=C070FKK6GBT

Preview Links

http://deploy-preview-5228--kongdeveloper.netlify.app/dedicated-cloud-gateways/managed-cache/#use-a-dedicated-cloud-gateway-managed-cache-in-a-custom-plugin

Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

[netlify]

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