(change) Replace log on variation by a log data exporter

[thomaspoignant]

Motivation

Currently, we are logging all the variations if a logger is provided.
Since this is not necessarily needed, we want to replace this with a Log exporter.

Requirements

A DataExporter that can Log with the same format as the one we have in the log.

By default we should have the same log format:

go-feature-flag/variation.go

Line 199 in bb1a2da

fflog.Printf(g.config.Logger, "[%v] user=\"%s\", flag=\"%s\", value=\"%v\"",