fix(resource): add plugin_configs support for fetch_latest_conf, fix ai-proxy-multi plugin with multi instance not support plugin_config_id

[chryancclee-boop]

When the ai-proxy-multi plugin uses plugin_config_id to reference a PluginConfig containing multiple instances, APISIX returns a 503 error: "fetch_latest_conf(): unsupported resource type: plugin_configs".

Root cause:

• pick_target() in ai-proxy-multi.lua calls resource.fetch_latest_conf() to fetch the parent configuration for load balancing and health checking.
• When referencing via plugin_config_id, the _meta.parent.resource_key is set to "/plugin_configs/".
• resource.fetch_latest_conf() previously only supported upstreams, routes, services, and stream_routes, causing it to reject "plugin_configs".

Why single instance works:

• Single instance scenarios bypass pick_target() entirely since no load balancing is needed, so resource.fetch_latest_conf() is never invoked.

Fix:

• bug: ai-proxy-multi plugin fails with plugin_config_id when multiple instances are configured #13456
• Add "plugin_configs" to the supported resource types in resource.lua.

Verification:

• Single instance + plugin_config_id: 200 (unchanged)
• Multi-instance + plugin_config_id: 503 -> 200 (fixed)
• Multi-instance + embedded plugins: 200 (unchanged)

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)

[Baoyuantop]

Please provide the corresponding test cases.