[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:

Add withTaskCancellationHandler to nextInvocation which calls close on cancel.
In LambdaChannelHandler.channelInactive resume continuation if state is waitingForNextInvocation
Added LambdaRuntimeClientTests.testCancellation

Result:

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

[adam-fowler]

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

[fabianfett]

My goodness. two comments have lingered here forever in pending. Just flushing those right now

[adam-fowler]

Re-wrote this. It is considerably simpler. Also fixed test to ensure it does check for cancellation

[fabianfett]

we should check that the spawned task returns with a cancellation error indeed!

[adam-fowler]

It doesn't return cancellation. Currently returns isInClosedChannel. Not sure how to get it to return the cancellation error as I'm just closing the connection and waiting for it to clean up. I could catch the error and then check Task.isCancelled and throw a Cancellation error if it is cancelled but that seems a bit of a hack.

[fabianfett]

This looks really good! Mind adding the test check.

[sebsto]

This looks really good! Mind adding the test check.

@adam-fowler ?

[adam-fowler]

Currently the test check will fail. There isn't an easy way to indicate the connection was closed due to cancellation without some re-structuring.

[sebsto]

Looks like we're good to merge then. Thank you for having confirmed