Add timestamptz option

[omgitzjoe]

Summary

Added an opt-in toggle to use timestamptz in Ash.Type.DateTime fields. Previous behavior is preserved for existing users.

Closes #264

Root cause

The default DateTime field uses timestamp (without time zones) and switching to timestamptz would fulfill best practices.

Changes

• Added an opt-in timestamptz datetime toggle.
• Added a test that verifies migration generator uses timestamptz with toggle enabled.
• Added a test that verifies the runtime Ecto uses timestamptz when the toggle is enabled.
• Added a test that verifies datetime values remain consistent when the timezone is changed.

How I tested it

• Added a describe block containing the three tests and setup.
• Test fails without this change: yes.
• mix credo passes with line-ending consistency warnings on the modified files.
• mix dialyzer passes locally.

Notes for reviewers

• Toggle is disabled by default to allow for existing behavior to continue

Contributor checklist

Leave anything that you believe does not apply unchecked.

• I accept the AI Policy, or AI was not used in the creation of this PR.
• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

[zachdaniel]

LGTM 👍 I see that its a draft, LMK (via a comment I don't get a notification when a PR stops being a draft) when its good to go on your end.

[omgitzjoe]

Right, it's should be good to go Zach.

[zachdaniel]

🚀 Thank you for your contribution! 🚀