Annotation processors to generate non-reflective converters to/from entities

[cjfuller]

Right now we convert between the typed kotlin representation of datastore models and the untyped datastore client's Entity type via reflection. We could instead use an annotation processor to generate non-reflective converters on-demand (which could potentially be a performance gain, especially for types with a lot of small properties?)

KA-internal issue: INFRA-2838