Support non-homogenous nested types in SchemaProvider implementations

Currently we can infer schemas for nested types, but only when they are homogenous (e.g. an AutoValue with an AutoValue field, or a Java Bean with a Java Bean field).

We should also support schema inference for non-homogenous types, so that schemas can be inferred for an AutoValue field with a Java Bean field.

Not doing so can lead to confusing errors like the one reported in https://lists.apache.org/thread.html/rc7c45ff8efd425fe545b6020c5b4250c99f6c788bf32a114708187a1%40%3Cuser.beam.apache.org%3E