Introduce IN (India) Region with Fixed-Date Public Holidays

[Chiragasourabh]

This PR introduces fixed-date public holidays for the IN (India) region.

This implementation only includes holidays that occur on fixed calendar dates. Many major Indian festivals (holidays) are based on lunar or solar calendars and thus vary each year depending on astronomical factors such as the position of the sun, moon, and other celestial elements. Those variable-date holidays will be added in a follow-up PR using appropriate date calculation methods.

[Chiragasourabh]

@ppeble , When you have time, would appreciate your review on this PR.

[ppeble]

Thank you!