Add additionalContext support to MLXLanguageModel

[noorbhatia]

Add CustomGenerationOptions struct to MLXLanguageModel with an additionalContext: [String: JSONValue]? property for injecting variables into Jinja2 chat templates

Usage

var options = GenerationOptions()
options[custom: MLXLanguageModel.self] = .init(
    additionalContext: ["enable_thinking": .bool(true)]
)
let response = try await session.respond(to: "Hello", options: options)