feat: add ai-lakera-guard plugin

[janiussyafiq]

Description

Add ai-lakera-guard, a new AI security plugin that integrates with Lakera Guard for LLM prompt-injection, jailbreak, and PII detection. Slots into the AI plugin chain at priority 1028, just below ai-aliyun-content-moderation.

This PR is the walking skeleton of a larger series — it lands the foundation and the request-side block path on openai-chat. Subsequent PRs will add the other three protocols, output-direction scanning, streaming, alert mode, fail_open, $secret:// integration, and user documentation.

The full schema shape is landed up front so later PRs add behavior only, not new fields.

Which issue(s) this PR fixes:

Partial implementation of #13291.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

User documentation will land in a later PR once the user-facing surface is fully wired.