Merge pull request #25582 from fengyuentau:dnn/dump_pbtxt

Current net exporter `dump` and `dumpToFile` exports the network structure (and its params) to a .dot file which works with `graphviz`. This is hard to use and not friendly to new user. What's worse, the produced picture is not looking pretty.
dnn: better net exporter that works with netron #25582

This PR introduces new exporter `dumpToPbtxt` and uses this new exporter by default with environment variable `OPENCV_DNN_NETWORK_DUMP`. It mimics the string output of a onnx model but modified with dnn-specific changes, see below for an example.

![image](https://github.com/opencv/opencv/assets/17219438/0644bed1-da71-4019-8466-88390698e4df)

## Usage

Call `cv::dnn::Net::dumpToPbtxt`:

```cpp
TEST(DumpNet, dumpToPbtxt) {
    std::string path = "/path/to/model.onnx";
    auto net = readNet(path);

    Mat input(std::vector<int>{1, 3, 640, 480}, CV_32F);
    net.setInput(input);

    net.dumpToPbtxt("yunet.pbtxt");
}
```

Set `export OPENCV_DNN_NETWORK_DUMP=1`

```cpp
TEST(DumpNet, env) {
    std::string path = "/path/to/model.onnx";
    auto net = readNet(path);

    Mat input(std::vector<int>{1, 3, 640, 480}, CV_32F);
    net.setInput(input);

    net.forward();
}
```

---

Note:
- `pbtxt` is registered as one of the ONNX model suffix in netron. So you can see `module: ai.onnx` and such in the model.
- We can get the string output of an ONNX model with the following script

```python
import onnx
net = onnx.load("/path/to/model.onnx")
net_str = str(net)
file = open("/path/to/model.pbtxt", "w")
file.write(net_str)
file.close()
```

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/dnn/include/opencv2/dnn/dnn.hpp

@@ -518,6 +518,14 @@ CV__DNN_INLINE_NS_BEGIN
          *  @see dump()
          */
         CV_WRAP void dumpToFile(CV_WRAP_FILE_PATH const String& path);
+        /** @brief Dump net structure, hyperparameters, backend, target and fusion to pbtxt file
+         *  @param path   path to output file with .pbtxt extension
+         *
+         *  Use Netron (https://netron.app) to open the target file to visualize the model.
+         *  Call method after setInput(). To see correct backend, target and fusion run after forward().
+        */
+        CV_WRAP void dumpToPbtxt(CV_WRAP_FILE_PATH const String& path);
+
         /** @brief Adds new layer to the net.
          *  @param name   unique name of the adding layer.
          *  @param type   typename of the adding layer (type must be registered in LayerRegister).