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 on the trait method)
- 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](https://openfeature.dev/specification/sections/flag-evaluation), [2.4 Initialization](https://openfeature.dev/specification/sections/providers)

Start by tracing provider registration through the provider mutator and locating the provider initialize trait method. Verify the optional bound domain is passed during registration, domain-scoped providers reject a second domain binding, and associated tests and documentation cover the new behavior.

Written by the indexing model from the issue text.