payment spec: change price per impression (or any other event) dynamically (AIP#6)

[Ivshti]

advertisers should be able to dynamically change the price per impression; this could be done by having them submit a validator message to both validators, which will update their eventRecorders to address the change

the original payment spec should contain a thresholds: see pricingBounds here

this would require changing eventAggregator, eventReducer and the channel routes so that existing recorders can be updated dynamically

for details on the schema, see AIP#6

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@samparsky start by going off the comment in the AIP that outlines the spec and PR-ing it to campaignSpec.md in adex-protocol

then, implement the date structures in Rust primitives and open a PR so we can review

Then proceed to implementing in the JS and Rust implementations, with comprehensive tests - unit tests for rule matching and integration test with a few compound rules matched together

[Ivshti]

closing cause it's a dupe of #157 ; actually this one was obviously the older one, but the newer one is better described

[Ivshti]

Re-opened cause we need a new spec for this (AIP6)

[Ivshti]

closing, it was done by #272