rego: don't parse inputs for wasm target (#3624)

opa.Eval() expects an input of type *interface{}; and for evaluations targetting
the wasm engine, we had been parsing the rawInput (*interface{}) into *ast.Value,
only to run ast.JSON() on it (going back to *interface{}) before passing it off
to the Wasm SDK.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>