Add a FabricTable API to get the next available fabric index.

This can be used during controller startup to map the fabric index (which is
allocated to the controller partway through startup and then used immediately
for storage keys) to the controller instance being started.

src/credentials/FabricTable.cpp

@@ -2080,4 +2080,19 @@ CHIP_ERROR FabricTable::GetFabricLabel(FabricIndex fabricIndex, CharSpan & outFa
  return CHIP_NO_ERROR;
 }
 
+CHIP_ERROR FabricTable::GetNextFabricAdditionIndex(FabricIndex * nextIndex)
+{
+ EnsureNextAvailableFabricIndexUpdated();
+ if (!mNextAvailableFabricIndex.HasValue())
+ {
+ return CHIP_ERROR_NO_MEMORY;
+ }
+
+ FabricIndex index = mNextAvailableFabricIndex.Value();
+ VerifyOrReturnError(IsValidFabricIndex(index), CHIP_ERROR_INVALID_FABRIC_INDEX);
+
+ *nextIndex = index;
+ return CHIP_NO_ERROR;
+}
+
 } // namespace chip

src/credentials/FabricTable.h

@@ -971,6 +971,14 @@ class DLL_EXPORT FabricTable
 #endif // CONFIG_BUILD_FOR_HOST_UNIT_TEST
  }
 
+ /**
+ * Get the fabric index that will be used for the next fabric that will be
+ * added. Returns error if no more fabrics can be added, otherwise writes
+ * the fabric index that will be used for the next addition into the
+ * outparam.
+ */
+ CHIP_ERROR GetNextFabricAdditionIndex(FabricIndex * nextIndex);
+
 private:
  enum class StateFlags : uint16_t
  {