Add Xet support to HTTPFS

[adsharma]

Summary

• add a self-contained XetFileSystem for xet:// Hugging Face model, dataset, and space URLs
• route xet:// paths through HTTPFS without requiring Xet client libraries
• handle Hugging Face resolve metadata, x-linked-size, signed redirects, and ranged reads
• add focused Xet URL mapping and glob tests

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.