Updates for Solvers

[JunShern]

We provide an update to our Solvers infrastructure

• Add a new README to onboard users wanting to work with solvers (beta)
• Creating a separate folder for registration: evals/registry/solvers
• Refactoring previous solver code to support reusability: NestedSolvers allow you to chain multiple solvers
• New solvers: FewShotSolver, SelfConsistencySolver, OpenAIAssistantsSolver
• A defaults.yaml for commonly reusable solvers
• Change abstract method for Solver action from __call__ to _solver so that task state is immutable