Add session key support to `topUpCDNPaymentRails` method

Side thought: you might also want to think about adding session key support to this, `msg.sender == info.payer`, maybe there's a case for a session key managing this - one frame I'm thinking about with session keys is as a GitHub Action API key in an environment variable, instead of a private key. If I give it access to just AddPieces to my data set, might there also be a case for having it top up my CDN service if it can also look at my usage and get some feedback about progress? Or should this be something that the owner wallet would only want to do through some management interface ("oh, I'm low, this looks reasonable but I should top up").

_Originally posted by @rvagg in https://github.com/FilOzone/filecoin-services/pull/284#pullrequestreview-3333746710_