Store Timezone as Arc<str>

[tustvold]

Which issue does this PR close?

Relates to #3955

Rationale for this change

Timezones are very often constant over a large number of types, e.g. if part of a session global default timezone. Maintaining separate allocations for every timestamp is therefore incredibly wasteful.

I originally wanted to propose storing Tz but this led to two major issues:

• DataType: Ord and it isn't clear what this would mean
• It would prevent roundtripping unrecognised timezones

Switching to Arc<str> allows reusing the same allocation whilst also reducing the size of the variant

What changes are included in this PR?

Are there any user-facing changes?

[tustvold]

This, combined with #3965 and corresponding changes to the Union DataType we can get DataType down from 56 bytes to 24

[alamb]

This looks like a good idea to me.

To ease the API transition for people can we please add some doc strings / examples for how to construct such a variant? If you are into Arc<str> it seems obvious but I remember it was somewhat confusing at first for a Rust newcomer

Perhaps we could add convenience methods to construct these variants like

impl DataType {
  fn new_timestamp(timeunit: .., timezone: impl Into<Arc<str>>) -> Self {
  }

Or something

cc @maxburke and @jhorstmann and @viirya and @waitingkuo given this is a non trivial API change

[tustvold]

Perhaps we could add convenience methods to construct these variants like

I'd rather avoid these as it results in a slightly confused API surface, but I will add some documentation to the variant itself

[alamb]

I'd rather avoid these as it results in a slightly confused API surface, but I will add some documentation to the variant itself

Adding docs is a good start -- and we can always add a new method in the future if that would help