SNMP: Add plugin manifest

[cakekoa]

What does this PR do?

Fixes part of #3234.

• Adds a plugin manifest for the SNMP plugin

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?
• Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• Added relevant unit tests?
• Do all unit tests pass?
• Do all end-to-end tests pass?
• Any other testing performed?

  Tested by {Running the Monkey locally with relevant config/running Island/...}

• If applicable, add screenshots or log transcripts of the feature working

[shreyamalviya]

We should mention brute-force.

[mssalvatore]

I couldn't figure out a way to do this without making the description unnecessarily wordy. I'm not sure how important it really is. We can revisit and discuss options if you think it can still be improved.