Backend: Implement Context values for Segments

[khvn26]

Blocked on Flagsmith/flagsmith-engine#219.

DoD

As an API user,
When performing

• POST /api/v1/projects/{project_pk}/segments/
• PUT /api/v1/projects/{project_pk}/segments/{id}/
• PATCH /api/v1/projects/{project_pk}/segments/{id}/

When providing a value under $.rules[].conditions[].property which is an invalid JSONPath within the evaluation context object, it is assumed to be a trait. This can be simplified to any property that doesn't start with $..

As an API user,
When performing

• POST /api/v1/projects/{project_pk}/segments/
• PUT /api/v1/projects/{project_pk}/segments/{id}/
• PATCH /api/v1/projects/{project_pk}/segments/{id}/

I'm able to provide the following values under $.rules[].conditions[].property:

• $.identity.identifier
• $.identity.traits.{any trait name}
• $.environment.name.

Then,

As an API user,
When performing

• GET /api/v1/flags/
• GET /api/v1/identities/

I can observe the Core API correctly fill the condition values for these properties when presenting the context for the engine, and evaluate flags using the expected segments.