feat: As a user, I want the `limit-*` plugins can return the `error_msg` directly, so that HTML or other kinds of response can be returned

### Description

Currently, the `limit-*` plugins support configuring an `error_msg` field to let users customize the error response when requests are throttled. However, Apache APISIX always puts the `error_msg` into a JSON string. It's not reasonable if users want to configure an HTML page. So, `limit-*` plugins should just return the `error_msg` without any wrapping. If a user wants to return a JSON string, just let this user configure this JSON string. So the same for HTML or others.

Of course, we may need another field to let users to configure the `Content-Type` so that the client can distinguish the response correctly.

https://github.com/apache/apisix/blob/4e25e5b4acc2e8293787b8861d0b4c2715cdc3af/apisix/plugins/limit-count/init.lua#L289-L293

Start with the linked limit-count plugin code at apisix/plugins/limit-count/init.lua#L289-L293 and compare how its error_msg response is currently produced. Trace the related limit-* plugin behavior and define completion as returning the configured message without JSON wrapping, with an appropriate Content-Type option for HTML or other response formats.

Written by the indexing model from the issue text.