Add values_to_plot support to matrixplot and heatmap (issue #3842)

[giovanamotta]

This PR adds values_to_plot support for rank_genes_groups_matrixplot and rank_genes_groups_heatmap, addressing issue #3842.

I implemented a small wrapper module (_rank_genes_groups.py) that extracts the selected differential expression metric (scores, logfoldchanges, pvals, or pvals_adj) from adata.uns['rank_genes_groups'] and passes it to MatrixPlot / Heatmap via values_df.

No changes were required to the plot base classes; the new functions simply reuse the existing engines (matrixplot, heatmap, dotplot). This keeps the PR minimal and adds full parity with the existing dotplot behavior.

• Closes Support values_to_plot in pl.rank_genes_groups_matrixplot and pl.rank_genes_groups_heatmap (parity with dotplot) #3842
• Tests: not required (thin wrapper using already tested plotting code)
• Release notes: not necessary (small API addition)