Expose programmatic way of loading a model

[kibertoad]

According to official documentation, it is possible to instantiate casbin model programmatically instead of loading it from file. See https://casbin.org/docs/en/model-storage#load-model-from-code

Would you be open to a PR that would implement support for the same in fastify-casbin?

[simoneb]

This is already possible, the only issue is that the name of the option modelPath is kinda off, because in practice what you provide can be anything that newEnforcer supports as the first argument, including an instance of a casbin model initialized in code.

[kibertoad]

@simoneb Types are wrong then. Can I submit a PR that would fix it then?

[simoneb]

absolutely! you may want to rename the argument to model, too. it will require a semver major but it would be a more accurate name

[kibertoad]

@simoneb Implemented it in backwards-compatible way, but if you prefer to only keep one param, can drop the modelPath and remove all the compatibility logic.