ci: enable `ConfigTests`

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

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