Add Build Events streams to the Correlated Invocations view when available

This MR adds support for fetching event streams from bgd browser-backend, and displaying them in the Correlated Invocations view.

This provides more insight into Bazel builds (and builds from any other tools which might go on to implement the Build Events Protocol) when viewing them in the Correlated Invocations view.

The UI implemented in this MR does a very basic level of parsing of the event streams; just enough to be able to render the events nicely. This groundwork can be expanded on in future MRs to support a better summary of build information on the Correlated Invocations view.

Verification

See the steps documented in buildgrid!714 (merged)

Related issues

Closes #20 (closed)