Add Custom Task Type Support

[mjburtenshaw]

Problem

As a developer, I want to create Tasks using custom task types.

Solution

📖 Definition: In ClickUp's nomenclature, a custom task type is a kind of custom item. In like fashion, I used custom item to name the data structures and custom task type as instances of custom items.

Main Quest

• Add CustomItemId to TaskRequest in clickup/tasks.go.
• Add CustomItemId to Task in clickup/tasks.go.
• Document nuances in values as comments.

Side Quests

• Add a CustomTaskTypes service to facilitate programmatic assignment of Task types.

Validation

• I updated the Get Task test.
• I added a test file for the CustomTaskTypes service.
• I tested an implementation of my fork in a private project with success.

[mjburtenshaw]

@w-haibara san, your review is appreciated.

+@a-thug (major contributor) +@heldeen (has recent context)

Thank you, all!