[core-lro] Define a default delay

[sadasant]

Define a default delay method. We can use either core-http's delay after setting a default polling interval, or we can use a setTimeout and clean it when the poller is stopped.

The idea comes from Will! Here's how he presented it: #11717 (comment)

[ramya-rao-a]

Closing as we have a core-util package that we are working on to house common code like delay. See https://github.com/Azure/azure-sdk-for-js/blob/main/sdk/core/core-util/src/delay.ts