Context Values: Make `EvaluationResult.flags` a dictionary

[emyller]

The current list-based format has a couple drawbacks:

1. It requires a certain order in the list, which forces extra translation code in some languages; e.g. in PHP, because json_decode guarantees no order of JSON keys.
2. It's not consistent with other structures in the Evaluation Context and Result, which use hashmaps (dictionaries) keyed by, e.g. feature name.

Warning

This will demand:

• From ongoing SDK work: to delete translation code.
• From future SDK work: to celebrate less work to do.

Also need to update the spec: https://github.com/Flagsmith/flagsmith/blob/main/sdk/evaluation-result.json