Implementing a capture_feedback function

[zmitchell]

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I forked the repo to attempt to implement this function based off of the corresponding function in the Javascript SDK: https://github.com/getsentry/sentry-javascript/blob/136370c8d4f0c0cd9ec80b47ba2292406d0b4ba7/packages/core/src/feedback.ts#L8

From what I gather, the necessary steps are:

• Add a Feedback variant to the Context enum.
• Add a FeedbackContext struct that holds contact_email, name, and message.
• I'm not sure what to do about the type field in the Javascript Event. I see it in the Go SDK as well for their event type, but I don't see it in the Rust SDK's Event type.
• Add the Context::Feedback to the contexts field of the Event
• Call capture_event with this new Event.

I went ahead and implemented that locally, but it's showing up as an "unlabeled event" (and not displaying in the User Feedback section), even though the "Feedback" context is populated as shown in the screenshot below. I've also checked that the project this event is sent to has User Feedback enabled.

You can see what I've implemented so far here (there's no capture_feedback function, but I'm creating the event and calling capture_event manually in my own project): https://github.com/zmitchell/sentry-rust/commit/248de09e183481ae8c0186b246aec8fffcf22979