Include Features and Segments in the Evaluation context schema

[khvn26]

In order to implement Flagsmith/flagsmith-engine#234, we need to add the following properties to the Evaluation context schema:

• features: a map of feature keys to feature objects that include a feature name and feature environment default value(s) with its (their) respective weight(s).
• segments: a map of segment keys to segment objects that include a segment name, a list of segment rules, and a map of the segment's feature overrides.

Additionally, we need to add an EvaluationResult schema that describes the following object:

{
    "context": context, // context used for evaluation
    "flags": [] // a list of evaluated flag values
    "segments": [] // a list of segments that the context matched with
}

In addition to the above, we need to answer the following question: is it valuable to give the newly added 'features' and 'segments' context keys access to the segment rules, i.e. to allow the implementation of the following rules:

• $.segments.sysadmins.overrides.butter_bar IS NOT "Attention! Server Room Flooding Imminent!"
• $.features.meta_feature_controlling_certain_segments.enabled IS true