FIP: Per API call micropayments

[reisepass]

Proposal: Allow providers to price their services on a per API call basis, authenticate customer calls with signed micro transactions and let providers submit agregated transactions on-chain when ever is most gas efficeint.

Customers prepurchase a certain amount of credits with the provider. Every API call from the customer should include in the headers a signed transaction for the price of an individual call. At the end of the month or once the pre-purchased credits are used up the provider could submit all transcations in aggregate. For the aggregation EIP-4337 may be used but Livepeer style probabilsitic micropayments could be more efficient.