Improve the payload encoding efficiency

[alexmturner]

Currently, the payloads embedded in aggregatable reports are not as size efficient as they could be. Some possible changes to reduce the size are explored here.

However, changing the payload encoding would be a breaking change for anyone using the plaintext payload included when debug mode is enabled. After debug mode is no longer available (i.e. after third-party cookie deprecation), changing the encoding should be simpler as only the user agent and the aggregation service should require updates. However, on a more minor note, the encrypted payload size and version would also change.