This also adds a test to ensure we are capturing usage of $set/$set_once
at the top level of the event, as posthog-js uses this method.
This was initiated by the issue mentioned
[here](https://github.com/PostHog/posthog-js/issues/615).
* docs(capture): add spec for capture endpoints
This adds an OpenAPI specification for the capture endpoints. This is
useful for documenting the API, and further can be used e.g. in tests to
validate that both clients and the API are behaving as expected.
