TaskBuilder doesn't catch TaskContext returning unserializable types

[kanterov]

For instance, in BigQueryLookupOperator, BigQueryLookup should be serializable.

I see two alternatives:

• check type signatures, I don't know if there is a good way to fix it due to type erasure
• check that ProcessFnArg#get return serializable type, but it might require a further extension of the code around to give good error messages