feat: client evaluation context getter and setter

[karnyadavdev]

closes #500

Exposes evaluation_context as a property getter and setter on OpenFeatureClient to support getting and setting a client-wide evaluation context, matching requirements from the OpenFeature specification.

Added a dedicated unit test in tests/test_client.py to verify getting and setting.

[gemini-code-assist]

Code Review

This pull request adds getter and setter properties for evaluation_context to the Client class, along with a corresponding unit test. The reviewer suggests updating the setter to default to an empty EvaluationContext if None is passed, which prevents potential type annotation violations and ensures consistency with the class's initialization logic.

[gemini-code-assist]

To prevent self.context from becoming None (which would violate the EvaluationContext return type annotation of the getter and potentially cause issues for consumers of the client), we should default to an empty EvaluationContext if None is passed to the setter. This also aligns with the initialization logic in __init__.

Suggested change

	@evaluation_context.setter
	def evaluation_context(self, evaluation_context: EvaluationContext) -> None:
	self.context = evaluation_context
	@evaluation_context.setter
	def evaluation_context(self, evaluation_context: EvaluationContext) -> None:
	self.context = evaluation_context or EvaluationContext()