Add functionality to load the overlay layer in the image_collection_gif

[iamtekson]

Description

I am trying to find a way to load and add the overlay layer in the get_image_collection_gif() function. Currently, the function works great to generate the animation of the single imageCollection which is great. But we should provide functionality to add the vector boundaries (AOI) or add other overlay layers in the same gif.

Source code

I think this can be doable using the add_layer() function. I will raise the PR on this issue.