Add global poll interval flag

### What problem are you facing?
Configuring the polling interval is a common flag of providers. e.e. See of [Azure](https://github.com/crossplane/provider-azure/issues/282), [GCP](https://github.com/crossplane/provider-gcp/pull/344) and [AWS](https://github.com/crossplane/provider-aws/pull/741)

### How could Crossplane help solve your problem?
Implement the polling interval as part of this template so any new providers will have it build in. See also https://github.com/crossplane/crossplane-runtime/issues/253
Document the polling interval and other flags. See also https://github.com/crossplane/provider-aws/issues/808