Agent - 2.4.0

- Introduce `/decide` endpoint as a new primary interface for Decide APIs, that is for retrieving feature flag status, configuration and associated experiment decisions for users ([#292](https://github.com/optimizely/agent/pull/292)).

- For details about this Agent release, refer to our documentation page: https://docs.developers.optimizely.com/full-stack/v4.0/docs/optimizely-agenthttps://docs.developers.optimizely.com/full-stack/v4.0/docs/optimizely-agent. - Upgrade to use [Go SDK v1.6.1](https://github.com/optimizely/go-sdk/tree/v1.6.1). This adds support for OptimizelyDecision.

https://github.com/optimizely/agent/releases/tag/v2.4.0