v0.5.7 Initial Bfloat16 support

Highlight

• Lance adds initial bfloat16 support for Machine Learning workloads!

https://lancedb.github.io/lance/arrays.html

What's Changed

• fix: make write_impl private by @wjones127 in #1061
• [Rust] Convert between json / arrow schema by @eddyxu in #1064
• [Python] Expose schema and json conversion via Python SDK by @eddyxu in #1067
• Add bfloat16 extension array by @wjones127 in #1002

Full Changelog: v0.5.6...v0.5.7