RSpec integration and remove subprocess support
This MR adds the following
- Adds rspec tests for allowlist and tags features
- Fixes broken rspec tests that are borrowed from GitLab secret detection gem (monolith)
- Integrates rspec tests(as integration test) in the GitLab CI template.
- Removes support for subprocess as it will not be supported by gRPC. It instead uses Ruby Threads with total processor count as the default value. Subsequently, removed subprocess-related rspec tests.