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 init when it's registered via the provider mutator
• Extend the provider init 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 init entry points described in the issue, then trace how provider declarations are registered and bound to a domain. Add the optional domain argument and domain-scoped binding enforcement, with associated tests and documentation. Done means domain-scoped providers cannot bind to multiple domains while existing providers remain compatible.