make all the internal function of ERC721Upgradeable virtual so they can be overriden

[adjisb]

🧐 Motivation
Sometimes you want to override methods like: _burn it seems that in general having internal methods virtual is a good idea.

Same thing about safeTransferFrom so we can add modifiers that limits the access or change the behavior.

[JhaMohan]

@adjisb can I work on this?

[Amxx]

_transfer, _mint and _burn are not virtual on purpose. This is to avoid inconsistency between calling an overriden version of these and the underlying _update function they are based on.

I suggest you override _update instead, as shown here