Add a debug text property to ASImageNode that allows users to show custom debug info

[nguyenhuy]

Such as image loading time, or part of the image URL.

[nguyenhuy]

Yes, they're separated APIs but use the same underlying debug text node. While it can be useful to combine them and show at the same time, the combined information may be too much to be shown on smaller nodes?

[nguyenhuy]

@maicki Can you take another look when you have time? Thank you.

[maicki]

@nguyenhuy I agree the information density could be pretty high. We may should just assert to either show one or the other. Let's fix the conflicts and get it landed.

[CLAassistant]

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