Show timezone in "s3 ls" output

**Is your feature request related to a problem? Please describe.**
The timestamps in `s3 ls` output are shown in the user's timezone (rather than UTC), which is fine but the TZ isn't actually printed so unless the user knows that, they might incorrectly assume it is in UTC or just aren't sure and then need to do additional checking to find out what timezone is being used.

Current output (URIs were modified for privacy):
```
$ aws s3 ls s3://foo/bar
2020-04-26 05:26:32 6736 bar
```

**Describe the solution you'd like**
Explicitly print the timezone or offset in the timestamps.

Example:
```
$ aws s3 ls s3://foo/bar
2020-04-26 05:26:32 -07:00 6736 bar
```

Above is with a -7 hour offset (time in Los Angeles, CA).

**Describe alternatives you've considered**
Could also print `PDT` or `America/Los_Angeles` but I think offsets are a little more clear. Could also switch to ISO 8601 or some other more standard date/time format.

**Additional context**
`aws-cli/1.18.68 Python/3.7.5 Linux/4.4.0-31-generic botocore/1.16.18`