CON-2626 new exercise findspairs

subjects/findpairs/README.md

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+## Find Pairs
+
+### Instructions
+
+Write a program that finds all pairs of elements in an integer array that sum up to a given target value. The program should output a list of pairs, each representing the indices of the elements that form the pair.
+
+In this exercise you must take in consideration the following:
+
+- Ensure it's possible to have positive or negative integers in the array.
+- Ensure each element is used only once in a pair, although the element can be repeated in different pairs.
+- Allow for multiple pairs to sum up to the target value.
+- The output messages should follow the one given in the examples bellow.
+- Return the message **"No pairs found."** when no pair is present.
+- Return the message **"Invalid target sum."** if the target is invalid.
+- Return the message **"Invalid number: <number>"** if the number in the array is invalid.
+- For any input format that deviates from the specified format `"[1, 2, 3, 4, 5]" "6"`, the program will return an "Invalid input." error message.
+
+Let's consider the input `arr = [1, 2, 3, 4, 5]` and the target sum `targetSum = 6`. When we run the program, the `findPairs()` function will search for pairs in the array that sum up to `targetSum`.
+
+Here is some example of outputs:
+
+```console
+$ go run . "[1, 2, 3, 4, 5]" "6"
+Pairs with sum 6: [[0 4] [1 3]]
+$ go run . "[-1, 2, -3, 4, -5]" "1"
+Pairs with sum 1: [[0 1] [2 3]]
+$ go run . "[1, 2, 3, 4, 5]" "10"
+No pairs found.
+$ go run . "[-1, -2, -3, -4, -5]" "-5"
+Pairs with sum -5: [[0 3] [1 2]]
+$ go run . "[1, 2, 3, 4, 20, -4, 5]" "2 5"
+Invalid target sum.
+$ go run . "[1, 2, 3, 4, 20, p, 5]" "5"
+Invalid number: p
+$ go run . "[1, 2, 3, 4" "5"
+Invalid input.
+$ go run . "1, 2, 3, 4" "5"
+Invalid input.
+$
+```