Show Datapoints of PR triggered NEXMark jobs on NEXMark plots

Now, that BEAM-6011 is resolved we have two types of NEXMark jobs:
- periodic ones: they publish their results to "nexmark" BigQuery dataset
- jobs triggered from PR comments: they publish their results to "nexmark_PRs" dataset

What is more, periodic jobs display plot their results [here](https://apache-beam-testing.appspot.com/explore?dashboard=5647201107705856), [here](https://apache-beam-testing.appspot.com/explore?dashboard=5099379773931520), [here](https://apache-beam-testing.appspot.com/explore?dashboard=5163657986048000) and [here](https://apache-beam-testing.appspot.com/explore?dashboard=5647201107705856).

****The goal:****

We could modify the plots to have datapoints from PR runs. Alternatively, we could create a separate dashboard for pr triggered jobs (which option is better?)

****Extra, optional goal (requires more work):****

Such plots could be embedded to GitHub PR comments so that the feedback is visible on Github directly after job runs.

Imported from Jira [BEAM-6071](https://issues.apache.org/jira/browse/BEAM-6071). Original Jira may contain additional context.
Reported by: ŁukaszG.