Implement getRemainingExecutionTimeInMillis on DurableContext #290
Description
The regular Lambda context has a utility function getRemainingTimeInMillis() which will provide the remaining time before the active invocation will timeout. Customers use this utility to approximate how long they have left in their lambda functions, which may influence business logic in their code.
We should implement an equivalent getRemainingExecutionTimeInMillis() on the durable context for the execution timeout. This will allow customers to perform similar checks for durable executions, across multiple lambda invocations.
We can do this by using the timestamp of the EXECUTION event in the execution state. This event is generated when the execution is started and should be accurate to measure against.