Create KnownBuilderInstances function for ExternalTransformRegistrar

Rely on instances of ExternalTransformBuilder rather than on class objects in ExternalTransformBuilder. This is in anticipation of implementing cross language transforms using the SchemaIOProvider interface, which requires a SchemaIO specific to the IO.

Additional context of larger effort:

https://docs.google.com/document/d/1ic3P8EVGHIydHQ-VMDKbN9kEdwm7sBXMo80VrhwksvI/edit#heading=h.x9snb54sjlu9

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