Show Datapoints of PR triggered NEXMark jobs on NEXMark plots

[kennknowles]

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, here, here and here.

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. Original Jira may contain additional context.
Reported by: ŁukaszG.