Return the battery object in endpoint /batteries (closes #2045)

changelog/70_UNRELEASED_xxxx.xx.xx.md

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+> ⚠️ xxxBREAKING CHANGESxxx
+
+> ❗ xxxImportant upgrade informationXXX
+
+### New feature: xxxx
+
+- xxx
+
+### Stock
+
+- xxx
+
+### Shopping list
+
+- xxx
+
+### Recipes
+
+- xxx
+
+### Meal plan
+
+- xxx
+
+### Chores
+
+- xxx
+
+### Calendar
+
+- xxx
+
+### Tasks
+
+- xxx
+
+### Batteries
+
+- xxx
+
+### Equipment
+
+- xxx
+
+### Userfields
+
+- xxx
+
+### General
+
+- xxx
+
+### API
+
+- The Endpoint `/batteries` now also returns the corresponding battery object (as field/property `battery`)