Add `key_events` to oss-directory

[ryscheng-mobile]

What is it?

For each project, we probably want to keep track of key events, for example:

• feature launch
• some incentive program
• etc etc

Then we can import these as a mart somewhere that's like key_events_by_project_v1, which can just be markers that we plot on a time series metric.

[ryscheng-mobile]

I guess it wouldn't hurt to do this also for collections