Replace `String` engine parameters with `impl AsRef<str>`

Hey 👋

Most Engine struct parameters that accept strings in their parameters mainly use String, which requires unneccessary heap allocations in scenarios where a simple &'static str will do. For instance, Engine::add_policy's example may be written to the following assuming that impl AsRef<str> is used instead of String; note how to_string() has been removed:

let mut engine = Engine::new();

let package = engine.add_policy(
   "test.rego",
   r#"
   package test
   allow = input.user == "root"
   "#)?;

assert_eq!(package, "data.test");

I am happy to contribute this change myself if you guys want to go ahead with the suggestion.

Start with the Engine API and the Engine::add_policy documentation example. Audit the other Engine parameters that accept String, then review the relevant tests and API compatibility. Done means string literals can be passed without to_string() wherever the change applies, with tests and documentation still passing.