Add time_zone support for user upsert

[amrani]

This PR adds time_zone parameter to the Users.upsert method, allowing you to set user time zones for better scheduling and localization.

Changes

• Added time_zone parameter to Users.upsert
• Introduced UNSET sentinel value to support clearing optional fields
• Added prepare_params helper to properly handle nil vs omitted parameters

Usage

# Set time zone
EmbedWorkflow::Users.upsert(key: "user-123", time_zone: "America/New_York")

# Clear time zone
EmbedWorkflow::Users.upsert(key: "user-123", time_zone: nil)

API documentation: https://docs.embedworkflow.com/reference/users/upsert-user