add log capture event
Right now, the Run view and the compute log manager have an implicit agreement about the structure of compute logs. The run view assumes that every run's compute logs are captured by step, and adds links to the raw compute logs in each step start event.
In order to have different granularity of compute log capture, this diff adds a distinct log capture event, along with a log key to query by. This should allow us to emit a log capture event per step to maintain the existing per-step capture behavior, but also add a per-process capture event, along with a distinct log key.
Test Plan: bk
Reviewers: alangenfeld, dish, sandyryza, cdecarolis
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D7939