[Feature]: Implement Grayscale function in helpers.c

[harshsankhla18]

So, what is it about?

In a grayscale image, the image saturation should be zero.

Hint: Convert each pixel in the image to grayscale by averaging the red, green, and blue color values.

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[saumya1317]

assign me this @harshsankhla18

[harshsankhla18]

Okay, Use the algorithm mentioned in readme.md ,if needed.