fix: bitmap index large size cause panic error

[982945902]

template <>
size_t
BitmapIndex<std::string>::GetIndexDataSize() {
    auto index_data_size = 0;
    for (auto& pair : data_) {
        index_data_size +=
            pair.second.getSizeInBytes(false) + pair.first.size() + sizeof(size_t);
    }
    return index_data_size;
}

like this code , index_data_size type will atomic infer as int type that not as excepted size_t, when size is large, it will overflow, so i change it to size_t.

in my case Cardinality is about 90000000, the index_data_size will outrange of int max 2,147,483,647, casue return size is very verry big like 18446744073709551615, next alloc memory will throw exception.

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