feat(datafusion): add BlobDescriptor SQL helper functions

[QuakeWang]

Purpose

DataFusion already had BlobDescriptor serialization and resolution support, but SQLContext did not expose the Java-compatible path_to_descriptor and descriptor_to_string SQL APIs. Queries using either the canonical names or sys.* aliases therefore failed as unknown functions.

Brief change log

• add path_to_descriptor and descriptor_to_string with sys.* aliases
• register both functions in SQLContext::new
• match Java serialization, string formatting, NULL, and invalid-input behavior
• document the SQL APIs

Tests

• cargo fmt --all -- --check
• cargo clippy --all-targets --workspace --features fulltext,vortex -- -D warnings
• cargo test -p paimon --lib blob_descriptor
• cargo test -p paimon-datafusion --test blob_descriptor_functions --test blob_tests --test sql_context_tests

API and Format

Adds four SQL function names and Display for BlobDescriptor. No storage format change.

Documentation

Updated docs/src/sql.md.