Add utils_spacelift_stack_config data source. Use gorotines for concurrent YAML stacks processing

[aknysh]

what

• Add utils_spacelift_stack_config data source
• Add examples/data-sources/utils_spacelift_stack_config example
• Use gorotines for concurrent YAML stacks processing
• Directly convert infrastructure YAML stack configs into Spacelift stack configs

why

• Speed up YAML stack processing (in many cases, from tens of minutes to tens of seconds)
• Go binary is much faster than Terraform on converting infrastructure stacks into Spacelift stacks when the complexity of computation is O(n^2) or O(n^3) (as we have in terraform-spacelift-cloud-infrastructure-automation)
• Use goroutines and WaitGroup to process a list of infrastructure YAML stack configs concurrently to speed up processing
• The old implementation was too slow - Spacelift terraform plan on about 260 infrastructure YAML stacks took 40-50 minutes
• In the new implementation, which does all processing concurrently (in parallel on all CPU cores) and directly converts infrastructure stacks into Spacelift stacks, Spacelift terraform plan takes about 17-20 seconds on 260 YAML stacks
• Backwards compatible with the previous releases

Plan: 1697 to add, 0 to change, 0 to destroy.

[mcalhoun]

Overall LGTM, I just left one question about possibly timing out the .lock() call within the goroutine