ConstNode GetAsXXXX methods refactor

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This issue is going to to Refactor ConstNode's GetAsXXXX methods, e.g. `GetAsString()`, `GetAsDouble` etc, since it is hard to know if these GetAsXXXX methods go well or not. So we have to re-design the methods as follows:
```
bool GetAsInt(int* output);
bool GetAsString(std::string* str)
```

Also, we will support GetAsString for types like `kBool`, `kDate`, `kTimestamp`.
1. kBool: `true` -> `"true"`, kBool: `false` -> `"false"`
2. kTimestamp: `1590115420000` -> `"1590115420000L"`
3. kDate:
```
Date date(2020, 05, 22);
to_string(date.date_)
```
need to discuss what `kDate` string looks like. `yyyy-mm-dd` pattern string will be far more reasonable.

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Look for ConstNode class in the codebase, likely in a header file. Examine existing GetAsXXXX methods to understand their signatures and usage. The refactor changes return types to bool and uses output parameters. Also need to implement GetAsString for kBool, kDate, kTimestamp types, deciding on string format for kDate. Check for any existing tests for ConstNode to update.

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