[Bug]: Schema providers of different types don't nest.

### What happened?

For example, if I have a class annotated with `@DefaultSchema(AutoValueSchema.class)` and it contains a field that is annotated with `@DefaultSchema(JavaFieldSchema.class)` it will claim that field has a row type with no fields.

One should be able to nest and re-use schema-aware classes without having to know or worry about how they're constructed, or at the very least this should be a clear error rather than a silent dropping of all fields.

### Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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