test: add trigger for e2e testing

[naugtur]

This is a work-in-progress attempt at triggering a workflow from a neighboring repo.

Discussion:

• it'd probably be easier to pull the other repo in a workflow in this one, but all the mess that's being installed in endo-e2e-tests to verify ecosystem compatibility would be nice to keep away from the CI in this repo.
  • that is unless we'd run it in a job that's explicitly allowed no access to any secrets (also future ones) etc.
  • if we mitigate the risks, it might be more comfortable to have the workflow here and pull the e2e repo in that.
• it seems the PAT token I'm testing with now is insufficient. Needs write permissions to endo-e2e-tests repo.
  • I can't create a scoped PAT to repos in endojs