accept timezone for timestamps

[julian7]

What type of PR is this?

• feature

What this PR does / why we need it:

When TimestampFlag is used with implicit timezone, default time.Parse behavior is assumed, which is parsing time in some timezone, and provide a UTC time. I haven't tested it myself, but I've read somewhere this works just with local timezone on windows, but it does this in UTC on linux. Either way, time.Parse assumes UTC times on linux and mac.

The suggested solution is to use time.ParseInLocation instead, and provide a *time.Location value for the default timezone.

This PR adds a Timezone member into TimestampFlag struct. Input parsing runs time.ParseInLocation when location is present, but keeps old behavior if Timezone is nil, to guarantee backwards compatibility.

Which issue(s) this PR fixes:

I haven't opened a separate issue for this PR.

Special notes for your reviewer:

I haven't included v2approve output.

Testing

This PR adds a new test, TestTimestampFlagApply_Timezoned (copied from TestTimestampFlagApply), which constructs a static timezone called PDT (UTC-7h), and expects the example UTC time provided in PDT timezone.

Release Notes

Add optional timezone (*time.Location) to TimezoneFlag to assume other timezones than UTC

[meatballhat]

Lovely! Thank you 🎉