Add Descriptor::IsSolvable() to distinguish addr/raw from others

src/script/descriptor.h

@@ -32,6 +32,10 @@ struct Descriptor {
     /** Whether the expansion of this descriptor depends on the position. */
     virtual bool IsRange() const = 0;
 
+    /** Whether this descriptor has all information about signing ignoring lack of private keys.
+     *  This is true for all descriptors except ones that use `raw` or `addr` constructions. */
+    virtual bool IsSolvable() const = 0;
+
     /** Convert the descriptor back to a string, undoing parsing. */
     virtual std::string ToString() const = 0;