Effect of Zr substitution on structural, electronic, thermodynamic and optical properties of $$(\textrm{HfO}_2)_{\text{p}}$$ clusters: a DFT study of $$(\textrm{HfO}_2)_{\textrm{p}}$$ and $$\textrm{Hf}_{\textrm{q}} \textrm{Zr}_{\text {r}} \textrm{O}_{2(\textrm{q}+\textrm{r})}$$ clusters