feat: Track response in error details

[tomasfarias]

When debugging apps, it's useful to have error messages. The problem is that we are only producing to app_metrics the message generated by reqwest (specifically, the message generated by this impl: https://github.com/seanmonstar/reqwest/blob/master/src/error.rs#L185).

This error message is useful, but to a limited degree: only status code and url are included in the message. Usually, the server's response can contain more information about what failed. As an example, see how the Django API includes more details on what failed in its default response:

curl -X POST -H "Content-Type: application/json" \ 
--data '{"api_key": "<redacted>", "batch": []}' https://app.posthog.com/batch
{"type": "validation_error", "code": "invalid_payload", "detail": "Invalid payload: All events must have the event name field \"event\"!", "attr": null}

This is valuable information we are currently suppressing in our handling of webhook errors. We should record this error message to be displayed in PostHog to assist with debugging.

Changes:

• WebhookJobError now is initialized directly from a WebhookRequestError, which is a new enum of errors that contains the response text.
• The display implementation of WebhookRequestError appends the response contents, if any.

Tests:

Included a unit test to check the error string contains the response body.

[tomasfarias]

Notice we are using a reference to response here when matching as error_for_status would have taken ownership of the response, and we need to keep ownership if we intend to return it.

[tomasfarias]

Had to rebase to pull in fix for broken dependency in #32

[tomasfarias]

Not handling these at the moment. hook-worker just defaults to an empty response on error.