When using default retry settings we should get that default from the specific service options

[aozarov]

As service may provide a specific exponential back-off requirements in its SLA.
E.g. for Datastore see this.

We should add a protected method in ServiceOptions to return the defaultRetryParams which
will be used when user didn't explicitly provided one. Default implementation should return RetryParams.defaultInstance().

We should override this method in services that provide their own specific retry settings such as Datastore and Storage.