Add `x-strake-warning` metadata header for queries under degraded licenses

[banickn]

Context: In crates/server/src/flight_sql.rs, when a query executes and the license state is Degraded, we currently only log a warning via tracing::warn!. We want to also send a grpc metadata header back to the client.

Task:

1. Locate StrakeFlightSqlService::execute_and_stream in crates/server/src/flight_sql.rs.
2. Find the match arm for LicenseState::Degraded (around line 139).
3. Update the code to append an x-strake-warning header to the Response object returned at the end of the function, with the text "Executing query in degraded license state".
4. Ensure it still logs the warning as well.