Allow adding Plotly toolbar buttons (streamlit#9190)

## Describe your changes

This PR fixes an issue where the configuration of `modeBarButtonsToAdd`
for the plotly toolbar was not respected.

## GitHub Issue Link (if applicable)

- Closes streamlit#9134

## Testing Plan

- Added e2e test.

---

**Contribution License Agreement**

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project are made under the Apache 2.0 license.

e2e_playwright/st_plotly_chart.py

@@ -49,7 +49,7 @@
     use_container_width=True,
     theme="streamlit",
     # Also test custom toolbar modification:
-    config={"modeBarButtonsToRemove": ["zoom"]},
+    config={"modeBarButtonsToRemove": ["zoom"], "modeBarButtonsToAdd": ["drawline"]},
 )
 
 # Candlestick Chart

frontend/lib/src/components/elements/PlotlyChart/PlotlyChart.tsx

@@ -445,6 +445,7 @@ export function PlotlyChart({
             }
           },
         },
+        ...(config.modeBarButtonsToAdd ?? []),
       ]
     }