Engine V7 compatibility

[khvn26]

After #143 is merged, remove all imports and references to flag_engine package, except get_evaluation_results interface introduced in Flagsmith/flagsmith-engine#234.

The list of imports to remove:

flagsmith/flagsmith.py:9,6: from flag_engine.environments.models import EnvironmentModel
flagsmith/flagsmith.py:10,6: from flag_engine.identities.models import IdentityModel
flagsmith/flagsmith.py:11,6: from flag_engine.identities.traits.models import TraitModel 
flagsmith/models.py:6,6: from flag_engine.features.models import FeatureStateModel
flagsmith/offline_handlers.py:3,6: from flag_engine.environments.models import EnvironmentModel

This should include a backwards-incompatible change to OfflineHandler signature. Right now, I believe that offline handlers should return an EvaluationContext dictionary, but it's a subject to discussion during further scoping of the issue.

[matthewelwell]

Regarding the change to the offline handler, here is some hastily written code to describe the planned changes.

import json

from abc import ABC, abstractmethod

from flag_engine.types import EvaluationContext


class BaseOfflineHandler(ABC):
    @abstractmethod
    def get_evaluation_context(self) -> EvaluationContext:
        """
        In most cases, this should return an evaluation context with no identity. 
        The identity will be merged into the context at evaluation time. 

        There may be a use case for including the identity in the offline evaluation 
        context - this would serve as a default, and could still be overridden at 
        evaluation time. 
        """
        raise NotImplementedError()


class LocalFileHandler(BaseOfflineHandler):
    def __init__(self, environment_document_path: str):
        with open(evaluation_context_path) as evaluation_context:
            self.evaluation_context = json.loads(evaluation_context.read())

    def get_evaluation_context(self) -> EvaluationContext:
        return self.evaluation_context

[matthewelwell]

From a documentation perspective, the plan is to document the migration guide in the release notes in github, and to link to the release notes from the documentation. We will also document pre v7 and post v7 in the offline handler section in the docs.