feat request(jwt-auth plugin): support fetch public key from jwks endpoint

### Description

Now in jwt-auth plugin configuration, the public key is a static string.
But in some scenarios, the public key may change over time, for example, it's exposed by a service's jwks endpoint.

It would be great to support such use cases.

Start by locating the jwt-auth plugin configuration and key-validation entry points, then inspect any existing plugin tests. Trace how the static public key is loaded and used, and define completion around accepting a JWKS endpoint and validating keys that can change over time.

Written by the indexing model from the issue text.