ci: enable ConfigTests

[muzarski]

This PR implements necessary utility functions to enable ConfigTests suite.

Changes

• implemented following util functions in Rust:
  • testing_cluster_get_port -> retrieves CQL port from cluster config
  • testing_cluster_get_connect_timeout -> retrieves connect timeout from cluster config
  • testing_cluster_get_contact_points -> returns a contact points string. Contact points are delimited with ,. If any of the contact points contained a nul byte, a null pointer is returned signifying an error appeared
  • testing_free_contact_points -> previous function allocates a string on the heap, and gives the ownership to the user. The user can free resources with this function.
• Adjusted cluster logic, when creating a contact points vector:
  • extra commas provided by user should be ignored
  • if user provided an empty string (after ignoring extra commas), the internal contact points vector should be cleared
• enabled ConfigTests test suite

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• PR description sums up the changes and reasons why they should be introduced.
• I have enabled appropriate tests in .github/workflows/build.yml in gtest_filter.
• I have enabled appropriate tests in .github/workflows/cassandra.yml in gtest_filter.