Fix time zone handling in index values and metadata

[G-D-Petrov]

Reference Issues/PRs

Fixes #1406 and #1827

What does this implement or fix?

• Unify the way time zones are handled for indexes and metadata, as much as possible
• Extend the support for time zone in the indexes/metadata to cover all cases supported by pandas
• Update the documentation to be more explicit about how/where time zones are supported

This is done by extending the current approach to decompose the timestamp into:

• its value - int
• its time zone info - str

This approach is beneficial as it:

• keeps the interface the same
• yields the most consistent results

The main problem is that we need to do some post processing to coerce the time zone info to be consistent (done here)

Any other comments?

An alternative approach was considered in a past PR here, but that had a couple of limitation:

• required an interface break
• was inconsistent because:
  • getting the offset value in some cases was incorrect inputs of type datetime
  • still needed to do some post processing to make sure that the time zone info is in a consistent format