Add Task domain

[gocastsian]

for now, just domain structure + minimum entity and crud usecases.

Consider Design in your implementation
https://www.figma.com/board/3V8OfM3w5y0AzUYB8TRjnZ/GoCasts-Leaderboard?node-id=0-1&t=fEXa9qBn62jFwK25-1