Add e2e tests

[emyller]

(...) a workflow that calls the locally-referred actions and verifies the result directly via the Flagsmith API. (...)

Originally posted by @khvn26 in #4 (comment)

Consider this will potentially spam the API with test artefacts. Ideally e2e testing should be pointed at environments with ephemeral data, e.g. a nightly-reset test environment, or a local instance of the API.