Factor out CLI code into new module, add "build" command

[naterubin]

This PR came out of the FilmDrop sprint in which I was given time to work on improvements to the STAC workflow developer experience. I was interested in something that would give more visibility into the content of a Task beyond what can be viewed in a Terraform definition, and an idea that came out of that is a FilmDrop task registry. The metadata command that gets added here is in service of that.

The other change is a refactoring of the CLI functionality to live in its own module and not necessarily be tied to a single task. This lets the run command execute one of many tasks registered to a CLI instance, which is helpful when packaging multiple tasks in a single Docker image.

A note on a breaking change that this PR introduces: with the run command now able to execute multiple tasks, a mandatory --task argument has been added. This means that existing invocations of this command will now fail. I had thought about a solution that would allow omitting the --task argument if only one task is registered to a CLI instance, but wasn't totally sold on that approach. Any discussion is welcome.

Related Issue(s):

None

Proposed Changes:

1. Factor CLI out of Task class into its own module.
2. Allow run command to run one of multiple tasks, as opposed to just one.
3. Add metadata command to output metadata of all tasks registered with CLI instance.

PR Checklist:

• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.