Add support for chrono FixedOffset

[willtrnr]

Adds a faker implementation for chrono's FixedOffset.

[llogiq]

Should we really allow minute-exact offsets? In the real world, there are mostly full hour offsets, with a scant few half hours strewn in.

[willtrnr]

You're right, I conflated two use cases and forgot to make the data realistic. I guess half-hour increments would be reasonable and I'll clamp the range to -12 to +14.

[cksac]

there are few offset at 15 / 45 mins from the list https://en.wikipedia.org/wiki/List_of_UTC_offsets#
I am not sure should we just random one from a fixed list?

[willtrnr]

I though about the exception cases (Nepal comes to mind), but considering we are just generating realistic data and not every possible cases I figured sticking to the slightly more regular 30min slices is good enough.

[llogiq]

The problem with the list is that it isn't fixed – countries don't change their time zones weekly but every once in a while, so we'd either need to embed a time zone database or go around and update our list every time that happens.