To add "Combination Sum-III" problem of leetcode no-216

[Amit-yadav099]

hello @JenilGajjar20
i want to contribute a code of combination sum -III in your list which is leetcode -216
as a newbie in contribution, kindly consider this and assign this work to me under hacktoberfest2024.

problem statement:-
Find all valid combinations of k numbers that sum up to n such that the following conditions are true:
Only numbers 1 through 9 are used.
Each number is used at most once.
Return a list of all possible valid combinations. The list must not contain the same combination twice, and the combinations may be returned in any order.

sample testcase:
Input: k = 3, n = 9
Output: [[1,2,6],[1,3,5],[2,3,4]]
Explanation:
1 + 2 + 6 = 9
1 + 3 + 5 = 9
2 + 3 + 4 = 9
There are no other valid combinations.