feat: Add attachment support to user feedback

[tustanivsky]

This PR adds the ability to attach files and binary data to feedback submissions allowing users to include screenshots, logs or other contextual information alongside their text messages.

The initial request to add attachment support for user feedback originated from the Unreal SDK:

• Screenshot as attachment along event / Feedback for Windows & Linux sentry-unreal#927

Here, a hint pattern is used (borrowed from the Android SDK) which relies on an opaque pointer to provide a standardized way to attach optional context or metadata to feedback without modifying its core value.

Example usage:

// Create the feedback (what to send)
sentry_value_t feedback = sentry_value_new_feedback(...);

// Create a hint
sentry_feedback_hint_t *hint = sentry_feedback_hint_new();
sentry_feedback_hint_attach_file(hint, "screenshot.png");

// Capture feedback with hint (both are freed automatically)
sentry_capture_feedback_with_hint(feedback, hint);

Example feedback with two image attachments.

Current implementation focuses on the feedback use case but it leaves room for an easy refactor to support additional APIs later on if needed.

[github-actions]

	Messages
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[jpnurmi]

Looks good at first glance, even though it feels like it's a lot of new API for a very specific use case. Since the goal is to add attachments to envelopes, I wonder if it would make sense to open up some of the existing but internal envelope APIs to make the same achievable in a more generic way. What do you think?

[jpnurmi]

Since the goal is to add attachments to envelopes, I wonder if it would make sense to open up some of the existing but internal envelope APIs to make the same achievable in a more generic way.

To elaborate a bit, I'd propose wrapping/exposing the missing parts from sentry_envelope.h so that the code becomes:

sentry_value_t feedback = sentry_value_new_feedback(...);

sentry_envelope_t *envelope = sentry_envelope_new();
sentry_envelope_add_feedback(envelope, feedback);
sentry_envelope_attach_file(envelope, "screenshot.png");

sentry_capture_envelope(envelope);

[tustanivsky]

Exposing internal envelope APIs sounds like a reasonable alternative to me.

However, if there are any concerns about doing so, the newly introduced hint type (perhaps with a more generic name like sentry_hint_t instead of sentry_feedback_hint_t) could serve as a public interface for attaching extra data to captured events as well.

Also, the before_send callback already accepts a hint parameter (currently defaulted to NULL) which might be repurposed for this as well - for example, in Android SDK the beforeSend handler provides a hint parameter that can be used to attach files to specific events. This contrasts with the current approach in Unreal SDK where attachments (like game screenshot) are added to the global scope during the on_crash hook processing so that crashpad can pick them up during uploading the event.