[Concept] Stateful Goroutines - add exercise templates

[zhravan]

Add exercise templates and tests for Stateful Goroutines.

Acceptance criteria:

• Add templates under internal/exercises/templates/:slug
• Add tests under internal/exercises/templates/:slug
• Add to internal/exercises/catalog.yaml
• Provide helpful hints in catalog.yaml

[aviralgarg05]

I would like to solve this issue for hacktoberfest

[zhravan]

Assigned it to you @aviralgarg05