Add Synk scanning

Snyk is a dependency vulnerability scanner.

It will require a token in order to operate. That token is ideally stored in Secrets Manager.

The Golden Pipeline should still deploy successfully if there is no token supplied -- it should just skip this check.

```{
"version": "0.2",
"phases": {
"install": {
"commands": [
"npm install -g snyk",
"pip install awscli --upgrade",
"pip install -r requirements.txt"
]
},
"build": {
"commands": [
"SNYK_TOKEN=$(aws secretsmanager get-secret-value --query SecretString --output text --secret-id snyk-auth-token --region us-west-2",
"snyk test",
"snyk monitor"
]
}
}
}
```

Start at the Golden Pipeline deployment flow and inspect how build commands and AWS Secrets Manager values are configured. Verify the Snyk commands and token handling described in the issue, then test both deployments with and without the snyk-auth-token secret; done means scanning runs when configured and deployment still succeeds when it is absent.

Written by the indexing model from the issue text.