Add enum.parse task to attack-of-the-trolls exercise

[KaranChadha10]

Description

This PR contains the following points:

• Add Task 5 to parse permission strings using Enum.Parse()
• Add 6 new test cases for permission parsing
• Update instructions.md with new task description
• Update hints.md with parsing guidance
• Update design.md with new learning objective and prerequisite
• Add generic-types as prerequisite concept

Closes #1467

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[ErikSchierboom]

I'm not sure about this PR. It requires adding generics as a prerequisite which makes this a harder exercise to access. Right now it teaches about flag enums and attributes and there is an existing enums exercise. Maybe we can use the non generic version and update the enums exercise to use parsing?