feat: introduce workflows and tasks

[galkleinman]

Feat

• Added Workflow type, which creates a new root span, and then propagating it's context.
• Added Task type, which can be created as part of workflow (represents a logical block/processing block). Creates a span in the context of the parent workflow.
• Added support for reporting Prompt/Completion in the context of Workflow/Task.

Fix

• sdk initialization is now repoted using log.printf instead of fmt.printf