Include process memory information

[bruno-garcia]

Events already include how much RAM the device has but no information about how much is being used.
More importantly how much RAM the application itself is claiming.

We can add it to error events, and transactions (after adding vitals #667)

[semuserable]

We can do that via Profiler Unity class. It has several methods that can help us. The only thing, where should we store such info in Sentry API? Should we use @event.Contexts.Device for that?

[bruno-garcia]

This is also a good candidate for our performance product. We'll be sending the Mobile vitals (frozen/slow frames and appstart) and other measurements will come