Add `Duration` support for `FixedBackOff` and `ExponentialBackOff`

[snicoll]

When creating a BackOff instance via RetryPolicy, we get support for Duration. However, when creating the BackOff implementation manually, we have to provide long value in milliseconds.

That's not a big deal but it'd be nice if the implementation themselves had support for Duration, potentially deprecating the long variant.

[sbrannen]

Duration support was actually added to FixedBackOff in conjunction with #35028 in 8b9e620 but removed in 15dd320.

Though, I'm in favor of supporting Duration in FixedBackOff and ExponentialBackOff, at least for the constructors and setter methods.

@jhoeller, thoughts?