[REVIEW] Expose silhouette score in Python

[wphicks]

Add Cython bindings for silhouette_score
Add Python functions corresponding to sklearn's silhouette_samples and silhouette_score
Update metric selection to make use of DistanceType enum

Resolve #645
Resolve #796

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[codecov-io]

Codecov Report

Merging #3164 (d8d17b0) into branch-0.17 (2a84c52) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           branch-0.17    #3164      +/-   ##
===============================================
+ Coverage        70.90%   70.98%   +0.07%     
===============================================
  Files              197      198       +1     
  Lines            15682    15725      +43     
===============================================
+ Hits             11120    11162      +42     
- Misses            4562     4563       +1     

Impacted Files	Coverage Δ
python/cuml/metrics/pairwise_distances.pyx	98.83% <ø> (ø)
python/cuml/metrics/cluster/__init__.py	100.00% <100.00%> (ø)
python/cuml/metrics/cluster/silhouette_score.pyx	100.00% <100.00%> (ø)
...l/_thirdparty/sklearn/preprocessing/_imputation.py	62.09% <0.00%> (-0.41%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2a84c52...d8d17b0. Read the comment docs.

[dantegd]

looks good so far, just have some comments and questions