dev/ci: define BK analytics test suite keys in agent manifests

Created by: bobheadxi

We currently fetch directly from gcloud secrets.

Docstrings in code has a TODO to move this to manifests link to https://github.com/sourcegraph/sourcegraph/issues/28469 but that doesn't seem to have any discussion regarding this, so I am opening a new issue to link to

cc @jhchabran