Make mintToTreasury virtual so it can be overridden

[davidlaprade]

This PR makes AToken's mintToTreasury function overridable.

Having this function be virtual will make AToken easier to extend, e.g. for grant-funded projects like the one I'm working on. It would also be consistent with the treatment of similar functions like AToken's mint, which is virtual:

aave-v3-core/contracts/protocol/tokenization/AToken.sol

Lines 87 to 92 in f3e037b

	function mint(
	address caller,
	address onBehalfOf,
	uint256 amount,
	uint256 index
	) external virtual override onlyPool returns (bool) {

[miguelmtzinf]

Make sense to me

[davidlaprade]

Thanks @miguelmtzinf!