Aggregatable report padding M119 update

Starting from M119 Canary, Beta, and Stable, to avoid the payload size being dependent on the number of contributions, the aggregatable reports will be padded with null contributions to a fixed length. This change will increase the size of the payload sent to the reporting endpoints, and the reporting endpoints using plaintext debug payloads will need to handle the null contributions added. This change is compatible with existing aggregation service versions.