refactor: create internal `_execute(...)` function in ERC725X

[CJ42]

What does this PR introduce?

In ERC725XCore, move all the logic of execute function inside an internal _execute(....) function. This will allow easier overriding of the public function execute(...) for custom logic.

Aside of that, removed the unnecessary override keyword for functions that are directly implementation of the IERC725X and IERC725Y interfaces.

[CJ42]

@YamenMerhi it's just a loop in the end, and use already the _setData(...) from the internal function. I would leave it as it is.

What would be your ideas of having:

function _setData(bytes32[] memory dataKeys, bytes[] memory dataValues) internal {
    for (uint256 i = 0; i < dataKeys.length; i = _uncheckedIncrement(i)) {
            _setData(dataKeys[i], dataValues[i]);
        }
}

[YamenMerhi]

Yes something like this, it's not a big deal (just for inheritance purposes).