Add xgboostserver

[sed-i]

(This PR replaces https://github.com/canonical/kubeflow-rocks/pull/2)

TODO:

• [ ] Automate out version: "1.16.0" and source-tag: "v1.16.0". At o11y we use this workflow.
• [ ] Simplify/remove the override-build section
• [ ] Figure out why starting XGBoostServer fails, raising an exception:

  File "/lib/python3.8/site-packages/sklearn/externals/joblib/externals/cloudpickle/cloudpickle.py", line 148, in _make_cell_set_template_code
    return types.CodeType(
TypeError: an integer is required (got type bytes)

[i-chvets]

xgboost server was de-prioritized. Closing this PR.