feat: token gated access on arbitrum sepolia with PPT token

[anurag-p6]

Summary

• Add optional pay-per-op PPT gated access under packages/core/src/gated-access/
• When gatedAccess is set on createMeshkitClient / Meshkit.init / init(), each upload / retrieve / pin transfers developer-priced PPT to their EOA or contract before IPFS
• Omit gatedAccess → free ungated ops (unchanged default)
• Ship Arbitrum Sepolia PPT preset (0x38c505EE3FDf02C0A041B08611aDB2F1d92DF410); Arbitrum One ready via tokenAddress override
• Docs only for where users buy PPT and that developers may swap PPT → USDC off-SDK (no DEX logic in code)