Add model and sample query for constants used in key specs

[knewbury01]

Goals:

• add support for and a query similar to: https://github.com/github/codeql/blob/main/swift/ql/src/queries/Security/CWE-259/ConstantPassword.ql
• add other misc key related support to modelling
• consider if additional key concepts can be used to improve query mentioned above (for example, right now a constant added to a spec is the main alert, but do we care more about if that key spec is also then used/used in a particular way?)

[knewbury01]

deprecated - was against an older version of base model, has been updated in #7