Support cron_tz in model_defaults for project-wide timezone configuration

[lafirm]

Feature Request

Add support for cron_tz in the model_defaults section of config.yaml to allow setting a default timezone for all models in a project.

Use Case

In projects where all models use the same timezone for cron scheduling, it's repetitive and error-prone to specify the timezone in every individual model definition. A project-wide default would significantly reduce configuration duplication and maintenance overhead.

Current Behavior

Currently, cron_tz must be specified in each individual model's MODEL() definition:

MODEL (
  name my_schema.my_model,
  cron: '0 * * * *',
  cron_tz 'Europe/London',
);

When attempting to set it in model_defaults, SQLMesh returns an error:

Error: 'cron_tz' is not a valid model default configuration key.
Please remove it from the model_defaults specification in your config file.

Desired Behavior

Allow cron_tz to be specified in model_defaults:

model_defaults:
  dialect: trino
  start: '2025-01-01'
  kind: VIEW
  cron: '0 * * * *'
  cron_tz: 'Europe/London'  # Should be valid here
  interval_unit: 'hour'