[FEATURE] Implement domain scoping

[beeme1mr]

Overview

A domain is an identifier that logically binds clients with providers, allowing multiple providers to be used simultaneously within a single application.

Domains were added to the spec in the following PR: https://github.com/open-feature/spec/pull/229

Tasks

### Tasks
- [x] [Requirement 1.1.3](https://openfeature.dev/specification/sections/flag-evaluation#requirement-113): The API MUST provide a function to bind a given provider to one or more clients using a domain. If the domain already has a bound provider, it is overwritten with the new mapping.
- [x] [Requirement 1.1.6](https://openfeature.dev/specification/sections/flag-evaluation#requirement-116): The API MUST provide a function for creating a client which accepts `domain`
- [x] [Requirement 1.2.2](https://openfeature.dev/specification/sections/flag-evaluation#requirement-122): The client interface MUST define a metadata member or accessor, containing an immutable domain field or accessor of type string, which corresponds to the domain value supplied during client creation.
- [x] [Requirement 3.2.2.3](https://openfeature.dev/specification/sections/evaluation-context#conditional-requirement-3223): The API MUST have a method for setting evaluation context for a domain.
- [x] [Requirement 3.2.2.4](https://openfeature.dev/specification/sections/evaluation-context#conditional-requirement-3224): The API MUST have a mechanism to manage evaluation context for an associated domain.
- [x] Update the `named provider` section of the readme to `domain`. See the JS SDK readme for an example: https://github.com/open-feature/js-sdk/tree/main/packages/server#domains

Reference implementations

Support was added in the JS SDK here and the Python SDK here.

[!NOTE] Some SDKs support named clients which predate domains. The domain term is intended to clarify intended behavior.