Add a gene_symbols parameter to `render_shapes` and `render_points`

[grst]

It would be great to have a gene_symbols argument to render_shapes and render_points, similar to what scanpy has in all (most?) plotting functions.

The goal is being able to use ensembl identifiers are var_names, while retaining the possibility to conveniently make plots by gene symbols, which are stored in another column in adata.var.

Possible alternatives: Instead of passing this to the function call, there could be a per-spatialdata configuration option which columns in .var to lookup when a color is specified.