Add Solana support to x402 in agents/mcp

[adlonymous]

The current implementation of x402 in packages/agents/src/mcp/x402.ts only supports base and base-sepolia. Solana support needs to be added to this file and any downstream locations to extend support beyond Base, starting with Solana to keep it up to sync with the rest of the x402 ecosystem like Vercel's x402-mcp