Add support for providing a default implementation for an interface method.

This enables providing a default implementation of an interface method. This method is defined on the Trait that is attached to the operation, and thus has all of the same constraints and properties as any other interface method. This allows for interface authors to provide a conservative default implementation for certain methods, without requiring that all users explicitly define it. The default implementation can be specified via the argument directly after the interface method body:

  StaticInterfaceMethod<
    /*desc=*/"Returns whether two array of types are compatible result types for an op.",
    /*retTy=*/"bool",
    /*methodName=*/"isCompatibleReturnTypes",
    /*args=*/(ins "ArrayRef<Type>":$lhs, "ArrayRef<Type>":$rhs),
    /*methodBody=*/[{
      return ConcreteOp::isCompatibleReturnTypes(lhs, rhs);
    }],
    /*defaultImplementation=*/[{
      /// Returns whether two arrays are equal as strongest check for
      /// compatibility by default.
      return lhs == rhs;
    }]

PiperOrigin-RevId: 286226054