Feat(aigw): headroom policy

[jbaross]

Description

Fixes #7078

References:

• https://konghq.aha.io/features/AI-136
• https://konghq.atlassian.net/browse/AG-1527
• https://github.com/Kong/ai-gateway-prds/blob/main/proposals/PRD-headroom-integration.md
• https://github.com/Kong/ai-gateway-prds/blob/main/proposals/PRD-cache-safe-prompt-compression.md
• https://docs.headroomlabs.ai/docs/proxy#post-v1compress and https://docs.headroomlabs.ai/docs in general

Preview Links

https://deploy-preview-7123--kongdeveloper.netlify.app/ai-gateway/policies/ai-prompt-compressor/

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).

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[jbaross]

The entity example here is drawn from the v1 plugin example and AHA tickets description of new parameters. We need this deployed on Konnect before it can be tested.

Both PRDs are important and there are some open questions:

• wrt the 'Prompt compression options' section, how does the conditionality work and does it carry over to Headroom
• where is lossy_backend: external declared and what other options does it have
• do we need an API reference or link to https://docs.headroomlabs.ai/docs/api-reference as a tab
• which if any of the existing parmeters effect headroom (timeouts etc)