feat(rpc): support Solana x402 drawdown - #56

Summary

• add SIWS authentication for Solana x402 drawdown sessions
• sign Ed25519 messages and encode signatures as Base58
• preserve the existing EVM SIWE path
• update SDK documentation and payment examples

Verification

• cargo fmt --all -- --check
• cargo test -p quicknode-sdk --lib --features rust,payments,payments-svm,payments-tempo
• cargo clippy --lib --tests --features rust,payments,payments-svm,payments-tempo -- -D warnings
• cargo build -p quicknode-sdk

The CLI integration PR consumes this local SDK change.