feat: Add Turkey calendar (Fixed holidays)

[ozankenangungor]

Part of #142

This PR adds the basic structure for the Turkey (Borsa Istanbul) calendar.

Changes:

• Added Turkey variant to Market enum in RustQuant_time/src/calendar.rs.
• Implemented fixed public holidays in RustQuant_time/src/countries/turkey.rs.
• Registered module in RustQuant_time/src/countries/mod.rs.
• Includes unit tests for fixed holidays.

Note: Religious holidays (Ramadan and Sacrifice Feasts) are currently excluded as they depend on the Hijri calendar logic, which can be added in a future PR.

[Copilot]

Pull request overview

This PR adds support for the Turkey (Borsa Istanbul) calendar to the RustQuant time library, implementing fixed public holidays based on the Gregorian calendar. The implementation follows the established pattern for adding new country calendars by adding a Market enum variant, implementing the holiday logic, and registering the module.

Changes:

• Added Turkey variant to the Market enum in calendar.rs
• Implemented is_holiday_impl_turkey function with 7 fixed public holidays
• Registered turkey module in countries/mod.rs

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
crates/RustQuant_time/src/countries/turkey.rs	New file implementing Turkey's fixed public holidays (New Year's Day, National Sovereignty and Children's Day, Labor Day, Youth and Sports Day, Democracy and National Unity Day, Victory Day, Republic Day)
crates/RustQuant_time/src/countries/mod.rs	Registered turkey module with public visibility
crates/RustQuant_time/src/calendar.rs	Added Turkey variant to Market enum and dispatched to is_holiday_impl_turkey in match statement

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[ozankenangungor]

Addressed the automated review comments regarding code style and visibility. Ready for review.