Add LambdaRuntime initializer with LambdaHandler directly with Codable support

[natanrolnik]

Add convenience initializer for LambdaRuntime to accept LambdaHandler instances directly

Motivation:

When using the Swift AWS Lambda Runtime with custom handler types that conform to LambdaHandler, developers previously had two options to initialize LambdaRuntime:

1. Manually wrap their handler with LambdaCodableAdapter and LambdaHandlerAdapter:

   let lambdaHandler = MyHandler()
   let handler = LambdaCodableAdapter(
       encoder: JSONEncoder(),
       decoder: JSONDecoder(),
       handler: LambdaHandlerAdapter(handler: lambdaHandler)
   )
   let runtime = LambdaRuntime(handler: handler)

2. Use a closure-based initializer that indirectly calls the handler:

   let lambdaHandler = MyHandler()
   let runtime = LambdaRuntime { event, context in
       try await lambdaHandler.handle(event, context: context)
   }

Both approaches are verbose and don't provide a clean, ergonomic API for the common case of initializing LambdaRuntime with a custom LambdaHandler instance. The closure approach also creates an unnecessary indirection layer, wrapping the handler in a ClosureHandler before adapting it, or using LambdaCodableAdapter.

Modifications:

Added a new convenience initializer to LambdaRuntime in Lambda+JSON.swift that accepts a LambdaHandler instance directly:

public convenience init<Event: Decodable, Output, LHandler: LambdaHandler>(
    decoder: JSONDecoder = JSONDecoder(),
    encoder: JSONEncoder = JSONEncoder(),
    logger: Logger = Logger(label: "LambdaRuntime"),
    lambdaHandler: sending LHandler
)
where
    Handler == LambdaCodableAdapter<
        LambdaHandlerAdapter<Event, Output, LHandler>,
        Event,
        Output,
        LambdaJSONEventDecoder,
        LambdaJSONOutputEncoder<Output>
    >,
    LHandler.Event == Event,
    LHandler.Output == Output

This initializer handles the wrapping of the LambdaHandler with the necessary adapters internally, matching the pattern already established for closure-based handlers.

Result:

Developers can now initialize LambdaRuntime with a LambdaHandler instance using a clean, direct API:

let lambdaHandler = MyHandler()
let runtime = LambdaRuntime(lambdaHandler: lambdaHandler)

This provides:

• Better ergonomics: More intuitive and less verbose API
• Consistency: Matches the pattern of accepting handlers directly, similar to how StreamingLambdaHandler can be used
• No extra indirection: Avoids wrapping the handler in an unnecessary ClosureHandler or LambdaCodableAdapter
• Type safety: Maintains full type inference for Event and Output types from the handler

[sebsto]

Love the idea and the simplification. Thank you @natanrolnik

@fabianfett any objection from an API point of view ?

[fabianfett]

Love it!

[sebsto]

A very welcome addition