c-s: Implement `insert` operation for user profiles

[muzarski]

Fix: https://github.com/scylladb/cql-stress/issues/91

This PR introduces a logic for predefined insert operation.

Changes:

• adjusted CLI to support insert operation for user command. Users can now choose to perform this operation (with a given sample ratio weight) via ops() parameter.
• implemented runtime logic for insert operation. As the name suggests, the operation simply generates a row and inserts it to database.
• updated README
• TODO: implement runtime tests in python

[muzarski]

I need to implement runtime tests for this feature, but apart from that, this PR is ready for review.