[core] Add support swift-service-lifecycle

[fabianfett]

The code on the main branch currently does not support Swift Service Lifecycle.

[sebsto]

We started the implementation with these two PR

• #459 : make LambdaRuntimeClient.nextInvocation() cancellable
• #456 : Add LambdaRuntimService and an example function

These two PR have unresolved issues.

• #459 is not Swift 6 compliant

/swift/lambda/swift-aws-lambda-runtime/Sources/AWSLambdaRuntimeCore/LambdaRuntimeClient.swift:343:38: warning: sending 'handler' risks causing data races; this is an error in the Swift 6 language mode
341 |                 defer {
342 |                     for continuation in array {
343 |                         continuation.resume(returning: handler)
    |                                      |- warning: sending 'handler' risks causing data races; this is an error in the Swift 6 language mode
    |                                      `- note: 'self'-isolated 'handler' is passed as a 'sending' parameter; Uses in callee may race with later 'self'-isolated uses
344 |                     }
345 |                 }

• #456 adds partial support for graceful shutdown. But the graceful shutdown flag is not propagated down to the LambdaRuntimeClient. Currently, this call cannot be interrupted when a graceful shutdown is requested.

[fabianfett]

Do we want to use package traits here?

https://github.com/swiftlang/swift-evolution/blob/main/proposals/0450-swiftpm-package-traits.md

cc @FranzBusch

[fabianfett]

I think we landed this with #490. @sebsto close this one?