Add unhandled_events field to native payload

[fractalwrench]

Goal

ANR detection delivered in #442 means that the number of unhandled events is no longer necessarily 1 in the latest version of the plugin. This requires a migration of existing payloads to fit the new struct, which adds an explicit unhandled_events field, rather than hardcoding 1.

Changeset

• Added a NOTIFY_UNHANDLED message to the native bridge, which is invoked whenever an unhandled event occurs, and ultimately increments report->unhandled_events
• Added unhandled_events field to bugsnag_report, which is serialised rather than '1'
• Created copy of existing struct in bugsnag_report_v1
• When reading a report, check the header. If it is v1, read into bugsnag_report_v1, then copy the fields over to bugsnag_report.

Tests

• Added test for session JSON serialisation in native reports
• Added test for payload migration
• Manually tested in an example app by:
  • Triggering a native crash with v4.12.0 and delivering a report
  • Triggering a native crash with v4.12.0, upgrading to a locally installed artefact, and then delivering the v1 report
  • Triggering a native crash with a local artefact to test the v2 report