Summary

Allow applications to supply a custom server selector to the Async C Driver.

Motivation

Applications with specific routing requirements need to influence server selection beyond the built-in read preference modes. The server-selection specification permits a custom selector, and this capability has been requested by users.

Acceptance Criteria

• An application-supplied server selector can be registered on a client.
• The selector is invoked during server selection.
• Behaviour is documented, including interaction with read preferences.
• Tests cover the added functionality.