Two components of the boundary of \(\text \textbf{I}_3\). (Deux composantes du bord de \(\text \textbf{I}_3\)) Symplectomorphism group of \(T^\ast (G_{\mathbb{C}} / B)\) and the braid group. I: A homotopy equivalence for \(G_{\mathbb{C}} = \mathrm{SL}_3 (\mathbb{C})\) 