Supply the bound domain to provider initialization

The spec now supplies the bound domain to a provider's initialize function, and adds an opt-in domain-scoped provider declaration that the API enforces by binding such a provider to at most one domain. This lets stateful providers (e.g. an OFREP static-context provider with a persistent cache) scope their state to a single, unambiguous domain.

What needs to happen

• Pass the bound domain (if any) to the provider's initialize when it's registered via the provider mutator
• Extend the provider initialize to accept the optional domain (non-breaking, additional optional parameter)
• Add a way for a provider to declare itself domain-scoped
• Enforce that a domain-scoped provider is bound to at most one domain, rejecting any attempt to bind an already-bound instance to another domain
• Associated testing and docs

Links

• Parent issue: https://github.com/open-feature/spec/issues/403
• Spec sections: 1.1 Flag evaluation, 2.4 Initialization

Start with the provider mutator and provider initialize API described in the issue, then trace how provider bindings are stored and validated. Add tests for optional domain passing, domain-scoped declarations, and rejection of a second domain binding; update the provider documentation. Done means existing providers remain compatible and the new constraints are covered.