data: Add Ethereum token metadata and performance benchmarks

Add reference data files:
1. eth_mainnet_token_metadata.csv (220k+ tokens)
   - Token contract addresses
   - Symbol, name, and decimals
   - Used for label joining in demo applications
   - Real production data from Ethereum mainnet

2. performance_benchmarks.json
   - Loader performance comparisons
   - Different loading methods benchmarked
   - Real-world throughput measurements
   - Configuration examples with results

Author: fordN