Add the ability to define a list

Why it matters

This feature enables ClickHouse to pass specific headers from the client's HTTP request to external HTTP authentication services, allowing these authenticators to use additional context or credentials embedded in the headers for more flexible and secure authentication workflows.

How to use it

Users can configure a list of headers to forward by specifying them in the ClickHouse settings for external HTTP authenticators. When an HTTP request is authenticated, the specified headers from the original client request will be included in the request sent to the external authenticator.