Support TypeDefinition options in beam.Combine()

Today, CombineFn's output type must be either concrete or a generic type that can be inferred from the input type. This doesn't work if the input type is struct{U, V} while the output type is V (where U and V are generic types).

Similar to ParDo, we could add TypeDefinition options support to Combine. This will bind generic types in the CombineFn to the specified concrete types at pipeline construction time.