Create a new engine to implement context values

[khvn26]

Blocked on Flagsmith/flagsmith-engine#235.

In order to separate the work, we want to create a new engine inside the repository which should pass against the update engine test data suite. The existing engine should be left intact, and removed in a subsequent pull request.

In order to do this, we should create a 'release' or 'parent' branch, which we will branch off of, for the separate pieces of work (see also #168)

• Support Evaluation Context and Evaluation Result schemas.
• Support JSONPath for selecting segment condition properties.
• Implement a single getEvaluationResult interface for the engine.
• Support JSON-encoded lists of strings for the IN condition string properties, falling back to comma-delimited.

This will require a major version of the SDK as we are making a breaking change to the offline handler implementation.