Adds mapping fields capability - #931

Non-nullable property 'Key' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

Dereference of a possibly null reference.

Dereference of a possibly null reference.

Possible null reference argument for parameter 'instance' in 'object? MiniExcelMemberAccessor.GetValue(object instance)'.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.

Possible null reference argument for parameter 'value' in 'List<MiniExcelColumnMapping?> ColumnMappingsProvider.GetColumnMappingFromValue(object value, MiniExcelBaseConfiguration configuration)'.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.

Possible null reference argument for parameter 'prop' in 'CellWriteInfo.CellWriteInfo(object? value, int cellIndex, MiniExcelColumnMapping prop)'.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.