Add Hook Data

[toddbaert]

Allow hooks to share data across stages within a single flag evaluation.

Specification References:

• 4.1.1: Hook context MUST provide hook data.
• 4.6.1: Hook data MUST be a structure supporting the definition of arbitrary properties.
• 4.3.2: Hook data is scoped to a specific hook instance.

Implementation Notes:

• Add hookData: [String: Any] to HookContext
• Each hook instance gets its own hook data dictionary per evaluation
• Enables use cases like OpenTelemetry span propagation between hook stages