Testing new and upcoming Glean SDK releases

[hackebrot]

We recently discussed in the cross-team meeting whether we could run the existing tests also with older version of the Glean SDK used by products as well as the latest main branch.

This issue is for exploring this idea further and scoping out the work required to achieve this.

[badboy]

Glean's main branch is the most up-to-date code and should be working at all times.
It uses the same version as the highest last released version, so it's not distinguishable from it by the payload.
The Python package can be built from source, all you need is the Rust compiler around (see Setup the Python Build Environment. That's easy to do in a Docker container (just keep in mind that it comes with increased build times).