Copy LogInput from audit package, add OptMarshaler interface (#6735)

Adds Type field and makes Request and Response interface{}.  

Add OptMarshaler interface for doing JSON marshaling with options.

sdk/logical/audit.go

@@ -0,0 +1,19 @@
+package logical
+
+type LogInput struct {
+	Type                string
+	Auth                *Auth
+	Request             interface{}
+	Response            interface{}
+	OuterErr            error
+	NonHMACReqDataKeys  []string
+	NonHMACRespDataKeys []string
+}
+
+type MarshalOptions struct {
+	ValueHasher func(string) string
+}
+
+type OptMarshaler interface {
+	MarshalJSONWithOptions(*MarshalOptions) ([]byte, error)
+}