Add Xet VFS support

[adsharma]

Summary

• mark xet:// as a non-local VFS path in the parent repo
• advance the extension submodule to include HTTPFS Xet support
• support xet:// Hugging Face model, dataset, and space URLs without requiring Xet client libraries

Extension PR: LadybugDB/extensions#3

Validation

• build/release/extension/httpfs/test/httpfs_xetfs_test --gtest_filter='XetFileSystemTest.*' passed 4/4
• live validation against ladybugdb/small-kgs: LOAD FROM 'xet://datasets/ladybugdb/small-kgs/main/kg_history/icebug-disk/csr_graph_nodes_person.parquet' RETURN count(*); returned 20

Build note: lbug_httpfs_extension builds successfully. The local build emits existing Homebrew OpenSSL macOS deployment-target linker warnings, but they are warnings only.