Add ability to add timeout duration to shutdown

[cijothomas]

All Providers have shutdown() method now, and it internally uses a hardcoded 5sec timeout to perform shutdown. (usually passed on to reader/processor/exporter).
Opening an issue to let user provide their own timeout.

Option1

fn shutdown(&self, timeout: Option<Duration>)

Option2

fn shutdown(&self)
{
self.shutdown_with_timeout(5secs)
}

fn shutdown_with_timeout(&self, timeout: Duration);

I am inclined to do 2, as it keeps backward compatibility, and we can add this anytime, not necessarily before 1.0. But given https://github.com/open-telemetry/opentelemetry-rust/pull/2573/files and changes in this area, it'd be better to club them in together.

[cijothomas]

Removing from 0.28 as option2 is additive change, and can be done post 0.28 as well.