Feature - returning response headers in SearchResult

[Z14tk0]

Summary:
This PR introduces support for returning HTTP headers in SearchResult, allowing the devs which use midpoint-client-java to read headers from response.

Use Case example:
When querying a paginated list of users (e.g., with offset=0 and maxSize=20), it's often useful to know the total count of available records. While there are several ways to return this (e.g., in headers, an envelope, or a separate endpoint), adding it to the headers is the least disruptive to the existing API.

Example Usage:

SearchResult<? extends ObjectType> searchResult = service.users().search().queryFor(UserType.class).get();
MultivaluedMap<String, Object> responseHeaders = searchResult.getHeaders();

This change enables fetching totalCount and other response metadata directly from headers without modifying the response body structure. (Note: Midpoint doesn’t currently return totalCount, but a subsequent PR will address this.)

Let me know if any adjustments or refactoring are needed.

Thank you,
Best regards