Add a function to search for key points in the image

[Giovaaanniii]

Text recognition will be based on key points. The key points are the interesting areas of the image. Interesting areas are those areas that are heterogeneous. For example, these can be angles, as there is a sharp change in intensity in two different directions

Conclusion: understand in which areas there is a sharp change in color intensity

[vsysoev]

Interface of the function should be something like:

input image
returned value keypoints

Plotting might be in some sort of tools which is used find_keypoints function

[vsysoev]

Function find_keypoints might be exported. Please pay attention to API, documentation and tests

[vsysoev]

Why these tabs in PR?

[vsysoev]

No need to plot keypoints in recog function. Function in library should not have such side effect.

Please define function something like this: (example) Comment section is very important. This is documentation of the function

def find_cropped_text(ctx, img: Image, txt: SearchedText,
                      skip: Optional[int] = 0, one_word: Optional[bool] = False):
    """
    Find text in image. Several passes are used.
    First time found area with text on image and then
    every area passed through recognition again to improve recognition results

    Args:
        ctx (Context): An object that contains information about the current
                    context.
        img (Image): image to search text in
        txt (SearchedText): text to search
        skip (int, optional): number of occurrences of the text to skip.
        one_word (bool, optional): flag if only one word has been searched.

    Returns:
        (roi, found):
            roi(ROI): region of interest
            found (bool): whether the text is found in the image

    """