CSI: fix timestamp from volume snapshot responses

[tgross]

Fixes #10670

Listing snapshots was incorrectly returning nanoseconds instead of
seconds, and formatting of timestamps both list and create snapshot
was treating the timestamp as though it were nanoseconds instead of
seconds. This resulted in create timestamps always being displayed as
zero values.

Fix the unit conversion error in the command line and the incorrect
extraction in the CSI plugin client code. Beef up the unit tests to
make sure this code is actually exercised.

---

After this patch, the CLI outputs look like the following.

Listing snapshots. Note these sort the reverse way I'd expect and want, but we'll come back to that in a separate patch.

$ nomad volume snapshot list -plugin aws-ebs0 -per-page 5
Snapshot ID             Volume ID     Size     Create Time                Ready?
snap-08391e1d0fe151035  vol-ffffffff  8.0 GiB  2017-02-08T07:14:09-05:00  true
snap-0d67b4f5b5355ba43  vol-ffffffff  8.0 GiB  2017-02-03T14:52:02-05:00  true
snap-0a65a2caffea0382e  vol-ffffffff  8.0 GiB  2017-02-03T14:09:45-05:00  true
snap-046281ab24d756c50  vol-033ca269  8.0 GiB  2017-02-02T18:57:19-05:00  true
snap-00de7e12fd08987c4  vol-0bf7e5b6  8.0 GiB  2017-02-02T18:44:49-05:00  true

Creating a snapshot:

$ nomad volume create ./input/ebs-volume0.hcl
Created external volume vol-0c7afdbcfa7a9d715 with ID ebs-vol[0]

$ nomad volume snapshot create 'ebs-vol[0]' tim-test-snapshot2
Snapshot ID             Volume ID     Size    Create Time                Ready?
snap-01b2cfe069ef183aa  vol-0c7afdbc  10 GiB  2022-03-22T16:52:15-04:00  false

[tgross]

(Switched this back to draft b/c there's a go.mod change that came up that I'd like to see if I can avoid here.) Fixed.

[shoenig]

LGTM; nanos are easier in scientific notation

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