feat(car_rental): add new exercise for rust exams

subjects/car_rental/README.md

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+## car_rental
+
+### Instructions
+
+Create a struct named `Car` and another one named `RentalBusiness`.
+
+The scope of the exercise will be to modify and operate on the `Car` in the `RentalBusiness` even if this element is not declared as mutable.
+
+### Expected Functions and Structures
+
+```rust
+use std::cell::Ref;
+use std::cell::RefCell;
+use std::cell::RefMut;
+
+#[derive(Debug, Default, PartialEq, Eq)]
+pub struct Car {
+    pub color: String,
+    pub plate: String,
+}
+
+#[derive(Debug)]
+pub struct RentalBusiness {
+    pub car: RefCell<Car>,
+}
+
+impl RentalBusiness {
+    pub fn rent_car(&self) -> Ref<Car> {}
+    pub fn sell_car(&self) -> Car {}
+    pub fn repair_car(&self) -> RefMut<Car> {}
+    pub fn change_car(&self, new_car: Car) {}
+}
+```
+
+### Usage
+
+Here is a program to test your function:
+
+```rust
+use car_rental::*;
+use std::cell::RefCell;
+
+fn main() {
+    let car_rental = RentalBusiness {
+        car: RefCell::new(Car {
+            color: "red".to_string(),
+            plate: "AAA".to_string(),
+        }),
+    };
+
+    println!("{:?}", car_rental.rent_car());
+    println!("{:?}", car_rental.repair_car());
+
+    {
+        let mut car = car_rental.repair_car();
+        car.color = "blue".to_string();
+    }
+
+    println!("{:?}", car_rental.rent_car());
+
+    car_rental.change_car(Car {
+        color: "pink".to_string(),
+        plate: "WWW".to_string(),
+    });
+
+    println!("{:?}", car_rental.rent_car());
+
+    println!("{:?}", car_rental.sell_car());
+    println!("{:?}", car_rental.sell_car());
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+Car { color: "red", plate: "AAA" }
+Car { color: "red", plate: "AAA" }
+Car { color: "blue", plate: "AAA" }
+Car { color: "pink", plate: "WWW" }
+Car { color: "pink", plate: "WWW" }
+Car { color: "", plate: "" }
+$
+```
+
+### Notions
+
+- [std::cell::RefCell](https://doc.rust-lang.org/std/cell/struct.RefCell.html)
+- [Struct std::rc::Rc](https://doc.rust-lang.org/std/rc/struct.Rc.html)