Context Values: Engine V7 compatibility

[khvn26]

Once Flagsmith/flagsmith-engine#246 is released, upgrade Edge API to the new engine version.

As a general tactic, bump the dependency version and make sure the tests pass.

The changes should be identical to Flagsmith/flagsmith-python-client#150, with a notable exception of having to respect hide_disabled_flags.

[khvn26]

Let's aim to import the code from Python SDK rather than copying it over.