[FEA] Support for callable functions on cuML SVC and SVR

[rubenandrebarreiro]

Is your feature request related to a problem?

• Currently, it seems to be missing the possibility of build SVC and SVR objects
  with callable functions as kernel, similarly to what is possible with Scikit-Learn.

From Scikit-Learn documentation (https://scikit-learn.org/stable/modules/generated/sklearn.svm.SVC.html#svc) - to show the callable possibility:

• kernel{‘linear’, ‘poly’, ‘rbf’, ‘sigmoid’, ‘precomputed’} or callable, default=’rbf’

Describe the solution you'd like

• I wish I could use cuML to build SVC and SVR objects with callable functions as kernel.

There is any idea about if this feature will be available in the future?