fix: make Tempo fee-payer policy chain-aware

[brendanjryan]

Summary

• resolve the Tempo fee-sponsor policy by chain instead of a single hardcoded constant
• raise the default Moderato/testnet maxPriorityFeePerGas ceiling to 50 gwei while keeping mainnet at 10 gwei
• add an optional feePayerPolicy override on tempo.charge() so servers can tune sponsorship limits without patching mppx

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/mppx@342

commit: ce97ba4