Add configurable costs for variable-work operators (#13931)

[d-sonuga]

Closes #13931.

Previously, variable cost operators like memory.fill were charged a constant non-configurable amount of fuel per byte. This extends OperatorCost with a VariableOperatorCost table so embedders can configure costs for operators whose work depends on runtime operands.

[github-actions]

Label Messager: wasmtime:config

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