Is it rude to use a fork and spoon instead of hands.

It is absolutely not rude or offensive in any way. Sri Lankans are incredibly understanding that eating rice and messy curries with your right hand is a specific skill that takes practice. Almost every single local restaurant even tiny street side shacks keeps a supply of forks and spoons specifically for foreign tourists. Simply smile and ask the waiter for a spoon and they will happily provide one immediately without any judgment whatsoever.