[FR]: Make `Combine` generating custom types (CombineTo)

### Does the feature exist in the most recent commit?

I did not find that functionality.

### Why do we need this feature?

It would be more handy to write:
```c++
struct MyStruct {
int a;
std::string b;
};

ParamGenerator gen = CombineTo(Values(1, 2, 3), Values("a", "b", "c"));
```
Than:
```c++
struct MyStruct {
int a;
std::string b;

using TupleT = std::tuple;

MyStruct(const TupleT& t): a(std::get<0>(t)), b(std::get<1>(t)) {}
};

ParamGenerator gen = ConvertGenerator(Combine(Values(1, 2, 3), Values("a", "b", "c")));
```

### Describe the proposal.

Actually I have already developed it. https://github.com/google/googletest/pull/4727

In two words: I have introduced a new function `CombineTo` and refactored `CartesianProductGenerator` to support custom types instantiation.

Maybe it is better to rename to `CombineAs`.

### Is the feature specific to an operating system, compiler, or build system version?

No

Start by reviewing pull request #4727, which the issue identifies as the existing implementation. Read the proposal for CombineTo and the requested CartesianProductGenerator refactor, including the possible CombineAs rename. Done means the feature and naming decision are resolved in the linked pull request.

Written by the indexing model from the issue text.