define "invoker" behavior

[keighrim]

(subtask of #3)

The evaluation invoker is the main player to manage a single evaluation process. Some features are also discussed in clamsproject/aapb-annotations#2 (comment).

At a high level, it should be able to handle

1. user input to start an evaluation process; this input doesn't have to manual, but can be a github actions dispatch upon a release of a new version of a CLAMS app via app-directory.
2. collection of AAPB GUIDs based on the evaluation dataset specified in the input
3. initiation of a CLAMS pipeline based on the GUIDs from the previous component and a pipeline configuration specified in the input
4. acquisition of result MMIF files once the pipeline is successfully finished (also see persisting machine annotations #10)
5. invocation of an "evaluator" specified in the input and obtain HTML with the evaluation result
6. automatic creation of a PR with the HTML file(s) from the previous component into github.io deployment branch/directory for manual review before publication of the evaluation results.