Add stopSession() and resumeSession() to public API

[fractalwrench]

Goal

Adds the ability to stop and resume sessions. This prevents the handled/unhandled error count from incrementing when a session is in the stopped state, which may be desirable if a user manually tracks sessions and does not care about crashes that occur in the background.

See previous PRs on Android and Cocoa for reference.

Changeset

• Updated bugsnag-android from v4.11.0 to v4.12.0
• Updated bugsnag-cocoa from v5.17.3 to v5.19.0
• Added StopSession and ResumeSession to Bugsnag.cs
• Added boolean flag to Session.cs to track whether a session is stopped or not
• Updated SessionTracker.cs so that CurrentSession returns null if StopSession is called, which prevents session information from being added to the session payload.
• Removed failing check_package task from Rakefile

Tests

• Added unit tests to verify the behaviour of StartSession, ResumeSession, and StopSession.
• Added stub classes to Tests/BugsnagUnity.Tests to allow for the construction of a SessionTracker in NUnit tests.
• Changed some classes to internal and used [assembly: InternalsVisibleTo("class")] to make classes visible to unit tests
• Manually verified that session information is not in an error payload when stopping a session, on Android + iOS.

[sazap10]

Seems fine having a quick sanity check through it