[flagd] Update selector handling for consistency with sync, evaluation, and OFREP services

[aepfli]

Context

This ticket follows open-feature/flagd#1814, which aims to normalize selector usage across all flagd services. Currently, selectors are handled differently in sync, evaluation, and OFREP services.

Requirements

• Reference: [FEATURE] Normalize selector handling across sync, evaluation, and OFREP services flagd#1814
• Ensure selector handling is consistent between go-sdk-contrib and core flagd, especially for clients and plugins interfacing with flagd.
• Ensure backwards compatibility, supporting the deprecated selector until the deprecation period ends.

Tasks

1. Audit all usages of selector in go-sdk-contrib (including SDK components interacting with flagd)
2. Update to prefer flagd-selector header when communicating with flagd services
3. Ensure backward compatibility where needed
4. Document selector handling patterns for maintainers and users
5. Prepare for future removal of deprecated behaviors

Benefits

• Predictable and consistent selector support when integrating with flagd
• Lower maintenance burden for migration in future major releases

Additional notes

• Work in alignment with the migration guidance and deprecation plan discussed in the core flagd ticket.