Consider sharing Udf/SdkFunctionSpec records via pointer

Coders are stored by pointer, because they are often repeated and a common source of huge pipeline descriptions.

We considered doing the same for all UDFs but decided not to, based on the logic that they are not as often identical and will rarely implement the equals() needed to actually share encoded versions.

However, in the presence of generated code, it is very likely that DoFns and CombineFns are repeated, and also much more likely that they have meaningful equals(), so there could be size savings.

None of this is terribly important for storage or transmission, but has more to do with arbitrary and small size limits that occur in some API frameworks or database column types.