`snowblock.Snowblock` API implementation

[arcticicestudio]

Epic: #33

Implement the pkg/api/snowblock.Snowblock interface (designed in #70) that represents a snowblock. It stores task data and metadata of a snowblock, validates it and dispatches the tasks to a registered TaskRunner (designed in #70) if one matches the task name, otherwise skips the unknown task.

[arcticicestudio]

Resolved in efdff96