apacheGH-44248: [Format] Add TimestampWithOffset canonical extension type (apache#48002)

### Rationale for this change

Closes apache#44248

Arrow has no built-in canonical way of representing the `TIMESTAMP WITH
TIME ZONE` SQL type, which is present across multiple different database
systems. Not having a native way to represent this forces users to
either convert to UTC and drop the time zone, which may have correctness
implications, or use bespoke workarounds. A new
`arrow.timestamp_with_offset` extension type would introduce a standard
canonical way of representing that information.

Rust implementation: apache/arrow-rs#8743
Go implementation: apache/arrow-go#558

[DISCUSS] [thread in the mailing
list](https://lists.apache.org/thread/yhbr3rj9l59yoxv92o2s6dqlop16sfnk).

### What changes are included in this PR?

Proposal and documentation for `arrow.timestamp_with_offset` canonical
extension type.

### Are these changes tested?

N/A

### Are there any user-facing changes?

Yes, this is an extension to the arrow format.

* GitHub Issue: apache#44248

---------

Co-authored-by: David Li <[email protected]>
Co-authored-by: Joris Van den Bossche <[email protected]>
Co-authored-by: Felipe Oliveira Carvalho <[email protected]>

Author: 4 people

docs/source/format/CanonicalExtensions.rst

@@ -544,6 +544,33 @@ Primitive Type Mappings
 | UUID extension type  | UUID                   |
 +----------------------+------------------------+
 
+.. _timestamp_with_offset_extension:
+
+Timestamp With Offset
+=====================
+This type represents a timestamp column that stores potentially different timezone offsets per value. The timestamp is stored in UTC alongside the original timezone offset in minutes.
+This extension type is intended to be compatible with ANSI SQL's ``TIMESTAMP WITH TIME ZONE``, which is supported by multiple database engines.
+
+* Extension name: ``arrow.timestamp_with_offset``.
+
+* The storage type of the extension is a ``Struct`` with 2 fields, in order:
+
+  * ``timestamp``: a non-nullable ``Timestamp(time_unit, "UTC")``, where ``time_unit`` is any Arrow ``TimeUnit`` (s, ms, us or ns).
+
+  * ``offset_minutes``: a non-nullable signed 16-bit integer (``Int16``) representing the offset in minutes from the UTC timezone. Negative offsets represent time zones west of UTC, while positive offsets represent east. Offsets normally range from -779 (-12:59) to +780 (+13:00).
+
+* Extension type parameters:
+
+  This type does not have any parameters.
+
+* Description of the serialization:
+
+  Extension metadata is an empty string.
+
+.. note::
+
+   It is also *permissible* for the ``offset_minutes`` field to be dictionary-encoded or run-end-encoded.
+
 Community Extension Types
 =========================