Variant field names not being ordered correctly via UTF-8 byte order

[rayokota]

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

The Variant spec requires the field ids in an object's header to be sorted by the
UTF-8 byte order of the field names, so a reader can binary-search them.
VariantBuilder sorted the fields — and Variant.getFieldByKey binary-searched them —
using String.compareTo, which orders by UTF-16 code units, not UTF-8 bytes.

The two orderings are identical for all field names in the Basic Multilingual Plane, but
they diverge for names containing supplementary-plane characters (U+10000 and above):
String.compareTo orders a leading high surrogate (0xD800–0xDBFF) before code points in
U+E000..U+FFFF, whereas UTF-8 byte order (and the spec) orders them after. Consequences:

• An object parquet-java builds with such keys has field ids sorted in an order that
  violates the spec.
• A spec-compliant reader (e.g. the Apache Arrow C++/Rust/Go Variant readers) binary-searching
  that object can fail to find fields.
• Conversely, parquet-java's own binary search fails to find a supplementary-plane key in an
  object produced by a spec-compliant writer.

The bug only surfaces when an object both contains a supplementary-plane key and is large
enough to take the binary-search path, so it has gone unnoticed.

Component(s)

Core