Add support ListSnapshots secrets

[bells17]

Hi, CSI supported ListSnapshots secrets in this PR.
So I want external-snapshotter to support ListSnapshots secrets.

ref: #152

[xing-yang]

There is a PR on this: #162. It was delayed because we were busy with the snapshot beta work. Hopefully we can get it in the next release.

[bells17]

@xing-yang Oh, Thank you.
I created PR...but should I close this?
(I did not notice that there was PR...)

[xing-yang]

Oh, I didn't notice that you've already created a PR. I'm checking with the author of the other PR and see if he is still interested in working on it.

Leave your PR there now. Let me sort it out first. Thanks.

[bells17]

I see. Thanks.

[xing-yang]

I have checked with the author of the other PR. You can keep your PR. I'll review it soon. Thanks for contributing!

[tdmanv]

@bells17 @xing-yang thank you!