Allowing users to control indexing of custom.*

We're seeing requests from customers to capture and index additional data. For example allowing some data to be analyzed and indexed to allow for full text search. We already have `labels/tags` and this covers situations where people want to have stuff indexed as `keyword` (and now also as numbers). We could extend `custom` to mean user-defined data and allow users to configure indexing through apm-server.yml, for example through something like `setup.template.append_fields`: https://www.elastic.co/guide/en/beats/filebeat/master/configuration-template.html

This would serve as an escape hatch to let users completely decide how they want to index custom things.

There's a consideration around RUM data, that people will be able to send up large amount of data that will get indexed if a certain field under custom gets indexed, but we already have similar potential problems with `tags`, so I don't think this adds additional dangers.