[core] Add cancellation handling for nextInvocation()

[adam-fowler]

Allow LambdaChannelHandler.nextInvocation to be cancelled.

Motivation:

If we want to use ServiceLifecycle with the lambda runtime the lambda runtime needs to be cancellable either via a ServiceLifecycle graceful shutdown or via Task cancellation. To avoid bringing in the ServiceLifecycle dependency this PR adds cancellation via Task cancellation handler.

Modifications:

Wrap withCheckedThrowingContinuation call with a withTaskCancellationHandler.
Added LambdaRuntimeClientTests.testCancellation

Result:

You can now cancel the runtime while it is waiting for the next invocation.

I am slightly concerned about this PR though (although I get no Sendability warnings/errors). The process inside the task withTaskCancellationHandler is not running on the event loop (which is why we need the context.eventLoop.execute {}) but I am allowed to set the handler state which I assumed would be isolated to the event loop. Would be good is @fabianfett or someone had a look at this before it was merged.

[adam-fowler]

This PR does not compile in swift 6 mode and I don't see an easy way around the issues.