Expose output_dir as a part of task context

[Nina-mvH]

Allow task definitions to access context.output_dir to save custom things to the tasks output directory.

For example, in a transcribe task, my task outputs may be .txt transcripts, but I may also want to save the model's raw output as .json, which I could do using context.output_dir.

Open questions:

• should users also be able to save things to the .tigerflow/output_dir?
• how to expose the pipeline's output_dir to the task? (logic likely pending resolution of Centralize .tigerflow at home directory #115 )