Add http_response_headers configuration for custom HTTP handlers

Why it matters

This feature enables users to customize the HTTP response headers returned by custom HTTP handlers, providing greater control over HTTP responses and improving integration with clients or proxies that require specific headers.

How to use it

Users can define the http_response_headers configuration in the ClickHouse server settings for their custom HTTP handlers by specifying the desired headers and their values. This allows the server to include those headers in the HTTP responses sent to clients.