This PR allows users to configure the threat models for their analysis by representing each threat model as an extension pack that can be added to the analysis. For example, if a user wanted to enable the "local flow sources" threat model, they would run codeql database analyze --extensionPack codeql/threat-local, or the Actions or VS Code equivalent. This avoids having to add a new configuration option in many different places.

Implementation notes

I moved the existing ExternalFlowConfiguration.qll file from Java into its own library pack, codeql/threat-models. This pack is language-agnostic. Any language that wants to consume threat models will depend on this pack, and every threat model extension pack will target this pack for extension.

For each of the high-level threat models currently supported, I added a new extension pack codeql/threat-<name>, where <name> is the name of the threat model. This extension pack just adds a single row to the supportedThreatModels predicate from codeql/threat-models, specifying the name of the threat model.

The threat model system in QL supports a hierarchy of threat models (for example, "files" and "environment" are both sub-threat-models of "local"). I haven't added extension packs for the low-level threat models yet, but we can add those later easily enough.