Add Insert from URL option to Gallery block

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The "Insert from URL" option was added to the Image block in gutenberg#40334.

The Gallery block nests Image blocks as child block to support adding multiple Images to a Gallery. Currently, the Gallery block only displays Insert from URL as an option for images items that have already been added to the Gallery. The Gallery block can be updated to allow "Insert from URL" behavior for all Gallery images.

https://user-images.githubusercontent.com/643285/169764320-43e4f020-4c20-4dcb-b6fc-cb489b4dc855.mp4