added new overlay classes for enhanced image robustness

[chakravarthik27]

This pull request introduces several enhancements and new features to the langtest/transform/image/robustness.py file. The changes focus on improving image transformation robustness and adding new classes for various image overlay techniques.

Enhancements to existing transformations:

• Image Text Overlay: Updated default text, font size, and position for better visibility and added transparency to the font color.
• Image Watermark: Added a default value for the watermark parameter to simplify usage.

New features:

• ImageRandomTextOverlay class: Introduced a new class to add random text overlays on images, enhancing robustness testing for visual QA tasks.
• ImageRandomLineOverlay class: Added a new class to overlay random lines on images, providing another method for robustness testing in visual QA.
• ImageRandomPolygonOverlay class: Created a new class to overlay random polygons on images, further diversifying robustness testing methods for visual QA.

Minor improvements:

• Reset mask in transform method: Ensured the mask is reset after each iteration to maintain consistency in transformations. [1] [2]