[18.0][IMP] date_range: add automatic "As Of Month" cumulative date range generation

[BhaveshHeliconia]

This PR adds automatic generation of cumulative monthly "As Of Month" date ranges to the date_range module.

Changes

New "As Of Month" date range type:

• Added a new date.range.type data record (date_range_as_of_month) with allow_overlap=True, used for cumulative monthly ranges (e.g., "As Of January 2025", "As Of February 2025", etc.).

Automatic generation via cron:

• Added generate_next_years_as_of_month() method on date.range.type that auto-generates cumulative ranges from January 1st of the current year through each month's end, for a configurable number of years.
• Added a monthly cron job (cron_generate_next_years_as_of_month) to trigger this generation automatically.

Settings configuration:

• Added a new res.config.settings model (res_config_settings.py) with a cumulative_month_range_years field (default: 5) to control how many years of ranges are generated.
• Added a settings view under General Settings → "Date Range Configuration".

Manual generation guard:

• Added a UserError in action_apply() of the date range generator wizard to prevent manual generation for the "As Of Month" type, since it is system-managed.

Search view improvement:

• Added a "Group by Type" filter in the date range search view.