Automatically pass variables to pipelines run via a multi-project trigger

Description

With #2772 (closed) we can pass explicit variables to triggers when using $CI_JOB_TOKEN. We could automatically pass "relevant" variables as part of this process.

Proposal

The trigger could automatically pass, in addition to CI_PIPELINE_SOURCE=pipeline, other information like project id, pipeline id, job id... from which in principle branch and commit could be obtained.

Links / references

Coming from #2772 (closed).

Documentation blurb

Overview

What is it? Why should someone use this feature? What is the underlying (business) problem? How do you use this feature?

Use cases

Who is this for? Provide one or more use cases.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.