Currently, the Trino Gateway's external routing rules engine allows routing decisions based on information sent to an external service, and the service can return a routingGroup and a list of errors.

This issue proposes enhancing the external routing service response to support a mechanism for the service to specify HTTP headers that the Trino Gateway would be able to set on the incoming request before forwarding it to a Trino backend.

Use Case:
A primary use case for this feature is to enable more sophisticated routing logic that can also enforce policies through HTTP headers. For example:

• Query Runtime Limiting: An external routing service could determine a query's characteristics and, based on those, set specific session properties via the X-Trino-Session header to enforce runtime limits on the Trino backend. This would allow dynamic policy enforcement at the routing layer.
• Dynamic Session Property Configuration: The routing service could set other session properties based on user, source, or other request attributes, providing a more flexible way to configure query execution context.

Proposed Response Format:
I propose extending the JSON response format from the external routing service to include an optional setHeaders field, which would be a JSON object where keys are header names and values are header values. For example:

{
  "setHeaders": {
    "X-Trino-Session": "query_max_execution_time=5m",
    "X-Trino-Client-Tags": "limited_runtime"
  },
  "routingGroup": "test-group",
  "errors": []
}

Considerations:
• Security implications of allowing an external service to set arbitrary headers. The Trino Gateway would need to carefully handle and potentially restrict which headers can be set.
• The interaction with existing header-based routing (e.g., X-Trino-Routing-Group).
• The order of header processing and potential conflicts.

Benefits:
This enhancement would provide a more powerful and flexible way to integrate external routing logic with the Trino Gateway, enabling advanced policy enforcement and dynamic configuration based on routing decisions.