Allow filtering aggregation keys

[johnivdel]

Currently, the API only supports filtering on key_pieces, rather than keys themselves. This means that you cannot control what aggregation keys get recorded for a trigger based on cross-site data, but instead only source side data.

[linnan-github]

We may consider allowing aggregatable_values field to specify a lists of values with filters, and similar to aggregatable_deduplication_keysfield, the first values in the list with matching filters will be used.

{
  "aggregatable_values": [
    {
      "values": {
        "a1": 1092,
        "a2": 1234,
      },
      "filters": ...,
      "not_filters": ...
    },
    {
      "values": {
        "a1": 1638,
        "a2": 4321,
      },
      "filters": ...,
      "not_filters": ...
    }
  ],
  ...
}