Variant getFieldByKey() on large objects silently fails if variant metadata is unsorted

### Describe the bug, including details regarding any error messages, version, and platform.

Variant method getFieldByKey(String key) looks up a key by simple walk if key count < 32, binary search if above that. But the binary search assumes the metadata is sorted. This is optional according to the format spec; there's a bit in the variant to indicate whether or not a variant's metadata is unsorted

the test `testUnsortedMetadataObject()` does test unsorted support -but it only checks on a small enough object that binary search isn't involved

### Component(s)

Core