[core] - move Timeout code from example to core

[sebsto]

The timeout.swift code from the Lifecycle example is duplicated in the the LambdaRuntimeClientTest (after merging #545)
There is also another code that implements a timeout, although less elegantly, in LambdaRuntimeTests

I propose

1/ to move the Timeout.swift file to the AWSLambdaRuntime target and reuse its mechanism in both tests and the lifecycle example
2/ to rename the function from timeout() to withTimeout() to be consistent with similar functions (withTaskCancellationHandler, withTaskgroup, withServer etc)

[sebsto]

Thinking about this, I will keep the timeout code duplicated between the example and the test target. That code is not necessary in the core runtime and we want to keep the binary as small as possible