Enable offline caching in MAUI

[mattjohnsonpint]

• Enables offline caching in MAUI
• Adds WebException to the exceptions that indicate network failure in the caching transport, as that's what thrown by Android when there is no Internet on the device.
• Adds an optional "network status listener" interface and option, which allows the caching transport to be more intelligent about whether it will attempt to send items from the cache to Sentry, based on network status.
• Tests for all the above.