feat(forward-auth): cache authorization decisions at the edge

[shreemaan-abhishek]

Description

forward-auth makes a synchronous HTTP call to the external authorization service on every request, which dominates its per-request overhead. This adds an opt-in per-worker cache of authorization decisions so repeated calls from the same caller can be served without hitting the auth service.

A new optional cache block:

"forward-auth": {
    "uri": "http://auth.internal/verify",
    "request_headers": ["Authorization"],
    "cache": {
        "key_headers": ["Authorization"],
        "ttl": 30,
        "include_method": false,
        "include_uri": false
    }
}

When present, an allow/deny decision is stored in a per-worker resty.lrucache and replayed on subsequent matching requests.

Because this is an authorization path, correctness is prioritized over speed:

• Never cache across identities. The cache key is namespaced per plugin conf id + version (via plugin_ctx_id, so config edits invalidate it) and built from the operator-declared key_headers values, optionally the method/URI, and — for POST — an md5 of the forwarded body. key_headers is required whenever caching is enabled, so there is no implicit "cache everything" default.
• Respects upstream cache-control. no-store / no-cache / private skip caching; max-age caps the TTL (and max-age=0 skips).
• Fails closed on transient errors. Auth-service 5xx responses are never cached.
• Faithful replay. Both allow (upstream headers) and deny (status, body, client headers) outcomes are reproduced on a hit, identical to the uncached path.
• Fully backward compatible: with no cache block the plugin behaves exactly as before.

Which issue(s) this PR fixes

N/A (enhancement).

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)

Tests

New t/plugin/forward-auth4.t counts actual auth-service invocations via a shared dict and asserts:

• schema requires key_headers and enforces ttl bounds
• identical requests hit the cache (1 auth call for N requests)
• a different identity is never served another's cached decision, and a cached deny stays a deny
• caching disabled still calls the auth service every time
• upstream Cache-Control: no-store is honored (not cached)
• include_uri keys on the request URI
• the POST body is part of the cache key

Existing forward-auth.t / forward-auth2.t / forward-auth3.t still pass unchanged.