[feat][clipboard-manager] Add "readImagePNG()" method

I am using the clipboard-manager plugin and readImage() in JS to get an image from the clipboard. The method returns the data as an uncompressed rgba bytes array which results in over 100MB for a screenshot of a UHD Desktop. This results in multiple seconds of wait time and is very inefficient.

A new `readImagePNG()` method could transfer the data as a PNG resulting a less data that need to be transferred.

The code for this already exists, before it was changed:
https://github.com/tauri-apps/plugins-workspace/commit/d57df4debe7c75cfbd6d6558fff1beb07dbee54c

It should be relatively easy to add a `readImagePNG()` as an additional option in the clipboard-manager plugin.
(Alternatively, the `readImage()` method could accept a paramater which format to return)