Add async LambdaHandler.shutdown

[adam-fowler]

Add async shutdown method to LambdaHandler protocol

Motivation:

LambdaHandler provides async interfaces for init and handle but not shutdown

Modifications:

• Add protocol requirement shutdown(context: Lambda.ShutdownContext) async throws to LambdaHandler
• Add default version that does nothing in extension
• Override ByteBufferLambdaHandler.shutdown to call the async version from LambdaHandler in extension

Result:

Can shutdown LambdaHandler using async methods eg

final class MyLambda: LambdaHandler {
    init(context: Lambda.InitializationContext) {
        self.httpClient = HTTPClient(eventLoopGroupProvider: .shared(context.eventLoop))
    }

    func shutdown(context: Lambda.ShutdownContext) async throws {
        try await self.httpClient.shutdown()
    }
}

[fabianfett]

I think we should add the async methods directly to the replacement for the shutdown methods in: #251

[tomerd]

agree with @fabianfett, lets get #251 over the finish line.

[adam-fowler]

Ok. I'm happy to close this in favour of #251 getting the async support