Support retrieving additional data from external services to enrich evaluation context

[yzx396]

When building a new feature and considering rolling it out to a large customer base in phases, there is requirement that certain customers who opt out any new features should be excluded permanently. The opt-out setting is usually stored in another config service which offers RESTful or GraphQL api access. There are other scenarios where we need to call services to get additional context data. Would it make sense to support this feature in flagd? @toddbaert

[yzx396]

#1262