Feature Request: Add New Country API Routes

[StPfeffer]

Description

• Introduced new API routes to retrieve country details using ISO 3166-1 codes (alpha-2, alpha-3, and numeric-3).
• Added new API routes to list the states of a country based on its ISO 3166-1 codes (alpha-2, alpha-3, and numeric-3).

Endpoints

Method	Endpoint	Description
GET	/api/v1/countries/alpha2/{code}	Retrieve details of a country using its ISO 3166-1 alpha-2 code
GET	/api/v1/countries/alpha3/{code}	Retrieve details of a country using its ISO 3166-1 alpha-3 code
GET	/api/v1/countries/numeric3/{code}	Retrieve details of a country using its ISO 3166-1 numeric-3 code
GET	/api/v1/countries/:countryId/states	Retrieve all the states of the country referring to the given ID
GET	/api/v1/countries/alpha2/{code}/states	Retrieve all the states of a country using its ISO 3166-1 alpha-2 code
GET	/api/v1/countries/alpha3/{code}/states	Retrieve all the states of a country using its ISO 3166-1 alpha-3 code
GET	/api/v1/countries/numeric3/{code}/states	Retrieve all the states of a country using its ISO 3166-1 numeric-3 code

Type of change

• [x] New feature (non-breaking change which adds functionality)

Checklist

• [x] I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• [x] I ran cargo fmt --all -- --check just before my last commit.
• [x] I ran cargo test just before my last commit and all tests passed.
• [x] I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).