support task repositories

[matthewhanson]

Core tasks are included in this Cirrus package (e.g., publish), as they are included in every deployment. However, tasks that are included in individual workflows (e.g., convert-to-cog) it often makes sense to version individually, and thus, also code version individually.

This is nearly always the case with batch tasks. A batch task repo would exist that is expected to publish a compatible docker image. However, with tasks that are written as Lambda functions (which can be used in batch with the cirrus-task-lambda-as-batch task), they would be included in the client-side repo for the cirrus deployment.

There should be a documented way to define tasks by linking to task repos. What would the task repos need to contain?

[jkeifer]

This is addressed by component plugins and/or lambda image support. See #139 and #104.