SparkCastOptions should represent timezone as Option<Tz> instead of String

[andygrove]

What is the problem the feature request solves?

In Comet's native cast logic, we currently pass timezones around as strings and use empty strings to represent lack of timezones. This results in repeated parsing of timestamps that could be avoided.

Describe the potential solution

Change timezone from String to Option<Tz>

Additional context

See SparkCastOptions, added in #1146.