Instrumenting Onboarding and Billing flows

[aj-codecov]

This follows our initial work to setup Amplitude for Codecov in Q4 #2890

The follow on work here is to fully instrument a couple flows in product:
#1 Our onboarding flow from signup to commit
#2 Our billing and cancelation flows

[aj-codecov]

@rohan-at-sentry handoff notes:
Spencer and Ajay are primary contacts here, Steven Eubank has also been engaged and helpful, we're in the process of building our onboarding instrumentation and will move to billing/cancelation next. No immediate action needed, but it's worth following along and making sure if you have instrumentation needs that the current work will meet them. There's opportunities here to get insight into how test analytics onboarding goes for example.