Consider API change to filters if analytics SDK uses Gjson

[colmsnowplow]

Configuring custom fields in filters for the v1 release, has an unintuitive API. This is mostly because of how the analytics SDK works under the hood. There's an issue to explore improving that: snowplow/snowplow-golang-analytics-sdk#38

If we do instrument gjson there, we can have a simpler jsonpath or jsonpath-like configuration of custom fields.