SwiftUI Picker - System photos picker

[etoledom]

Description

This PR implements the system photos picker for the Gravatar picker in SwiftUI.

Notes:

• As a demo, I have implemented both add image buttons: in-grid and bottom CTA.
• We are only using the UIKit photos picker, and not the new SwiftUI PhotosPicker, since the later only supports Photo Library, doesn't support cropping, and the selection seems bugged (made for multi-select only, though it can return only one).
• The interface of the ImagePickerView is similar to the one of SwiftUI's PhotosPicker, to facilitate a replacement in the future.
• After the selected image is uploaded to the backend, we add the local image to the cache, to avoid an unnecessary download.

Not included on this PR:

• Selector between Camera and Library.
• Possibility to inject custom images picker / cropper. (do we want this in the first release?)

Testing Steps

• SwiftUI demo app.

[etoledom]

@pinarol - Ready for another look 👍

• Moved the color to the design system declaration
  • The designs has the same blue for both light and dark modes.
• Moved the new struct views to their own files with previews.
• The CTAButtonView has variable title.
• Constants are back to be bounded to AvatarPickerView.
• Removed the wrong error handling from avatar upload.
• Few other small things.