Configuring User Agent for remote JWKS fetch

*Description*:
When using JWT authentication via SecurityPolicy and remote JWKS, envoy gateway fetches the keys from the configured endpoint. It looks like these calls are done with user agent `Go-browser`. It would be nice if this wa a bit more specific (e.g. `Envoy Gateway`) or even better, configurable for the user.

Start at the Go code path that fetches remote JWKS for JWT authentication configured through SecurityPolicy, and inspect where the current `Go-browser` user agent is set. Done means remote JWKS requests use a more specific or user-configurable user agent, with the behavior verified in the relevant tests if present.

Written by the indexing model from the issue text.