pubsub: add ability to pass timeout to ack

[jcking]

See #1979.

Something like this "should" work assuming the protoOpts are passed as is to gax, granted I haven't tested it yet. This adds an options argument to Subscription.prototype.ack that forwards options.timeout to gax. It preserves existing behavior and replicates the style of arguments orders that is consistent with the rest of the library.

Feel free to nit pick as this was quick and dirty while investigating another issue, I was in the neighborhood.

[jcking]

This is related to #1974 as well. Additionally I am not sure if we want to propagate the timeout option given at the creation of the Subscription?

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[stephenplusplus]

Sorry for the long delay on this one. I've added tests -- let's see what CI thinks!

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[coveralls]

Coverage decreased (-0.01%) to 99.912% when pulling d0accde on jcking:pubsub-ack-timeout into 688ac58 on GoogleCloudPlatform:master.

[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

😕 The bad news is that it appears that one or more commits were authored by someone other than the pull request submitter. We need to confirm that they're okay with their commits being contributed to this project. Please have them confirm that here in the pull request.

Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this state. It's up to you to confirm consent of the commit author(s) and merge this pull request when appropriate.

[coveralls]

Coverage increased (+6.0e-05%) to 99.924% when pulling ed4f719 on jcking:pubsub-ack-timeout into 688ac58 on GoogleCloudPlatform:master.

[stephenplusplus]

Thanks @jcking!