Add support to `gitlabktl` for parallel serverless builds using `parallel` syntax in CI YML

Problem to solve

Speed up build times for many serverless functions defined in serverless.yml by making it compatible with the parallel syntax in .gitlab-ci.yml.

Intended users

Further details

Proposal

The gitlabktl tool should be updated to understand the environment variables CI_NODE_INDEX, CI_NODE_TOTAL that are exposed by the parallel feature.

Permissions and Security

Documentation

Testing

What does success look like, and how can we measure that?

Links / references