Add Snyk scanning & monitoring

[ryan-wren]

Checklist

=========

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have checked for similar issues and haven't found anything relevant.
• This is not a security issue (which should be reported here: https://circleci.com/security/)
• I have read Contribution Guidelines.

Internal Checklist

• I am requesting a review from my own team as well as the owning team
• I have a plan in place for the monitoring of the changes that I am making (this can include new monitors, logs to be aware of, etc...)

Changes

=======

• Added vulnerability-scan job to the CircleCI config, to run Snyk scans and monitoring

Rationale

=========

The Snyk scans should be used to discover vulnerabilities in this project.

Considerations

==============

Why you made some of the technical decisions that you made, especially if the
reasoning is not immediately obvious

Screenshots

============

Before

Image or [gif](https://giphy.com/apps/giphycapture)

After

Image or gif where change can be clearly seen

Here are some helpful tips you can follow when submitting a pull request:

1. Fork the repository and create your branch from main.
2. Run make build in the repository root.
3. If you've fixed a bug or added code that should be tested, add tests!
4. Ensure the test suite passes (make test).
5. The --debug flag is often helpful for debugging HTTP client requests and responses.
6. Format your code with gofmt.
7. Make sure your code lints (make lint). Note: This requires Docker to run inside a local job.

[ryan-wren]

The original PR for these changes is here - #974 , but I messed up doing a rebase.

@vallieres I switched the logic as suggested to upload results from the Snyk scans on the dev branch. Here is the before screenshot:

And here is the after screenshot:

So, the results were uploaded to Snyk for this branch as expected. I'll delete the projects that were created from this branch. And I switched the logic back to only upload results on the main branch.

[vallieres]

LGTM!