Parallelize Commands

[aorinevo]

Introduction

Shepherd currently makes use of serialized calls to apply commands (e.g., checkout, apply, pr, push) to each repo. The purpose of this issue is to migrate these commands to execute in parallel to improve performance of each command execution.

Use Case

As an engineer, when I run Shepherd command against 100s of repos, I would like the commands execution to be parallelized to reduce execution time.

Acceptance Criteria

1. When a command is run (e.g., checkout), output behavior is the same except ordering.
2. Unit tests pass
3. e2e tests pass
4. Update documentation to include parallelization

POC

#852