Add ability to parallelize custom semgrep checks

[nbrahms]

Is your feature request related to a problem? Please describe. A user would like to run multiple custom semgrep checks in parallel in order to minimize check run time.

Describe the solution you’d like One of:

• Specify multiple check packs, each running in a separate semgrep process
• Automatically parallelize semgrep runs by file or check within Bento
• Automatically parallelize within semgrep