Introduce tipping mechanism for sponsor delegates/signers

[ilikesymmetry]

Bundlers get paid by the EntryPoint per ERC-4337's spec, but how does the entity that runs an automated gas policy service get paid? Current intuition is this use case will represent the majority volume of GAS-powered user operations and incorporating an incentive mechanism may aid in incentivizing more developers to compete at this layer and partake in protocol-native revenue.

One checkpoint thus far is that unwrapping GAS to pay these signers in ETH is vulnerable to blank user operations being submitted with the sole purpose of large tips to the signer, breaking the core invariant that GAS can only be spent on user transactions. One resolution to this is to reward delegates/signers with GAS, which they can choose to sell on the open market (albeit at a discount relative to ETH but that loss can be baked-in).