Allow to manually add upstream/downstream runs to have manual lineage metadata in the UI

[polyaxon-team]

Use case

From slack:
How can I manually add upstream/downstream runs to have full lineage via the UI?
We currently have this info in our configs but requires a few more steps for someone to search for the prior job uuid, I’d like to link it up via the upstream/downstream options in the UI

Feature description

We do not have the possibility to link runs (upstream/downstream) via UI, currently we do not expose that option as a manual API, the upstream/downstream tabs are automatically populated when the DAG runtime is used.

It would make sense to allow at least the API or Python Client to set the dependency graph manually.

run.set_downstream_of(UUID)