feat: Allow ending a session with a different status code

[ShawnCZek]

As of right now, the status code of the session cannot be modified through this SDK. This is a problem when errors or crashes are handled manually. Such sessions are then marked as "crash-free". This is especially problematic when the on_crash callback is used, which may discard processing the event further. This is a snippet from the inproc backend:

sentry-native/src/backends/sentry_backend_inproc.c

Lines 531 to 556 in 19df84a

	if (options->on_crash_func) {
	SENTRY_TRACE("invoking `on_crash` hook");
	event = options->on_crash_func(uctx, event, options->on_crash_data);
	should_handle = !sentry_value_is_null(event);
	}
	if (should_handle) {
	sentry_envelope_t *envelope = sentry__prepare_event(
	options, event, NULL, !options->on_crash_func);
	// TODO(tracing): Revisit when investigating transaction flushing
	// during hard crashes.
	sentry_session_t *session = sentry__end_current_session_with_status(
	SENTRY_SESSION_STATUS_CRASHED);
	sentry__envelope_add_session(envelope, session);
	// capture the envelope with the disk transport
	sentry_transport_t *disk_transport
	= sentry_new_disk_transport(options->run);
	sentry__capture_envelope(disk_transport, envelope);
	sentry__transport_dump_queue(disk_transport, options->run);
	sentry_transport_free(disk_transport);
	} else {
	SENTRY_TRACE("event was discarded by the `on_crash` hook");
	sentry_value_decref(event);
	}

This is the only place where the session may be marked with the crashed status code.

Because of these reasons, I propose making the session status a part of the public API and an option to end the session using that custom status code. Another possibility is making the session structure public like in other Sentry SDKs. However, that would be a greater change to support it properly in all places.

[loewenheim]

LGTM, but I would like a second opinion on this.

[codecov]

Codecov Report

Merging #801 (9438186) into master (19df84a) will decrease coverage by 0.02%.
The diff coverage is 86.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #801      +/-   ##
==========================================
- Coverage   81.19%   81.18%   -0.02%     
==========================================
  Files          53       53              
  Lines        6936     6947      +11     
  Branches     1111     1112       +1     
==========================================
+ Hits         5632     5640       +8     
- Misses       1194     1196       +2     
- Partials      110      111       +1     

[supervacuus]

CC: @markushi, @bitsandfoxes