feat: mlserver-sklearn rock

[i-chvets]

Design and implementation details are in canonical/seldon-core-operator#133

Summary of changes:

• Created rockcraft file for mlsever-sklearn rock according to best practices spec.
• Added unit test.
• Added tox.ini

Tested manually:

$ docker run mlserver-sklearn_1.2.0_22.04_1_amd64:rock exec pebble exec  bash -c 'export PATH=/opt/conda/bin/:/opt/mlserver/.local/bin:${PATH} && export PYTHONPATH=/opt/mlserver/.local/lib/python3.8/site-packages/:${PYTHONPATH} && source /opt/conda/etc/profile.d/conda.sh && source /hack/activate-env.sh /mnt/models/environment.tar.gz &&  mlserver start /mnt/models'
2023-06-30T19:09:12.508Z [pebble] Started daemon.
2023-06-30T19:09:12.525Z [pebble] POST /v1/exec 16.675242ms 202
2023-06-30T19:09:12.541Z [pebble] GET /v1/tasks/1/websocket/control 15.243491ms 200
2023-06-30T19:09:12.542Z [pebble] GET /v1/tasks/1/websocket/stdio 101.381µs 200
2023-06-30T19:09:12.542Z [pebble] GET /v1/tasks/1/websocket/stderr 211.268µs 200
2023-06-30T19:09:12.563Z [pebble] POST /v1/exec 16.031617ms 202
2023-06-30T19:09:12.580Z [pebble] GET /v1/tasks/2/websocket/control 16.109979ms 200
2023-06-30T19:09:12.581Z [pebble] GET /v1/tasks/2/websocket/stdio 143.794µs 200
2023-06-30T19:09:12.581Z [pebble] GET /v1/tasks/2/websocket/stderr 118.723µs 200
Environment tarball not found at '/mnt/models/environment.tar.gz'
Environment not found at './envs/environment'
2023-06-30 19:09:13,110 [mlserver.parallel] DEBUG - Starting response processing loop...
2023-06-30 19:09:13,110 [mlserver.rest] INFO - HTTP server running on http://0.0.0.0:8080
INFO:     Started server process [20]
INFO:     Waiting for application startup.
2023-06-30 19:09:13,120 [mlserver.metrics] INFO - Metrics server running on http://0.0.0.0:8082
2023-06-30 19:09:13,120 [mlserver.metrics] INFO - Prometheus scraping endpoint can be accessed on http://0.0.0.0:8082/metrics
INFO:     Started server process [20]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
2023-06-30 19:09:13,125 [mlserver.grpc] INFO - gRPC server running on http://0.0.0.0:8081
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:8080 (Press CTRL+C to quit)
INFO:     Uvicorn running on http://0.0.0.0:8082 (Press CTRL+C to quit)