v0.10.15: fix for LanceSchema pickling and empty directories

What's Changed

• fix: pickle LanceSchema nested fields correctly by @wjones127 in #2223
• fix: do not create directory when open an non-existing dataset by @eddyxu in #2215
• perf: build HNSW and quantization storage in parallel by @BubbleCal in #2196
• chore: remove extend_candidates option by @BubbleCal in #2217
• perf: avoid copying every vectors multiple times during building by @BubbleCal in #2216
• chore: search for enough candidates to build connections by @BubbleCal in #2218
• feat: add support v2 fragments on the scan path by @westonpace in #2213
• feat: allow loading extension vector index by @chebbyChefNEQ in #2221

Full Changelog: v0.10.14...v0.10.15