refactor: migrate to pure ESM dependencies for better Vite compatibility

[0xethll]

Summary

Migrate from CommonJS dependencies (fetch-retry, keccak) to pure ESM alternatives (ky, ethers built-in crypto) to improve compatibility with modern bundlers like Vite.

Problem

When using @zama-fhe/relayer-sdk in Vite projects, the package must be excluded from pre-bundling (optimizeDeps.exclude) because it contains CASM files. However, this causes Vite to fail when processing CommonJS dependencies within the excluded package.

Solution

• Replace fetch-retry with ky (pure ESM HTTP client with retry support)
• Replace keccak package with ethers' built-in keccak256, concat, and getBytes functions

Benefits

• ✅ Better Vite compatibility
• ✅ Eliminates CommonJS dependencies for improved tree-shaking
• ✅ Reduces package count (removes keccak, @types/keccak, fetch-retry)