Add ContainerClient exists method

[tasherif-msft]

Currently we have an exists() method on blob client. We should add a similar method on ContainerClient().

[pwr22]

Hi @tasherif-msft, any chance this is going to make it's way into a release soon?

[tasherif-msft]

This will be in our next release, which will be happening within the next week or so :)