Add front methods to json_pointers (implements #4889)

[trdesilva]

Added front, pop_front, and push_front methods to json_pointers in order to facilitate root-to-leaf traversals of JSON object trees. (#4889) These methods complement back, pop_back, and push_back, allowing json_pointers to be used as deques instead of just stacks. Example use case from the linked issue:

This is the pattern that I'm using for the case in the original post:

if(pointer.front() == INDIRECTION_TOKEN)
{
    pointer.pop_front();
    objectToPatch = fetchObject(pointer.front());
    pointer.pop_front();
    patchOperations[i]["path"] = pointer.to_string();
}
objectToPatch.patch(patchOperations);

More generally, pop_front() is useful for manually traversing the path specified by a json_pointer when you can't use json::operator[] on the entire pointer because you need to do some intermediate work during the traversal.

while(!pointer.empty())
{
    jsonObject = jsonObject[pointer.front()];
    doSomething(jsonObject);
    pointer.pop_front();
}

Also made a minor update in unit_json-pointer.cpp to use CHECK_THROWS_WITH_AS instead of CHECK_THROWS_WITH to conform with current Contribution Guidelines. Didn't have make available in Windows dev environment, so amalgamate was run manually and make pretty was skipped; hopefully CI can do this?

• The changes are described in detail, both the what and why.
• If applicable, an existing issue is referenced.
• The Code coverage remained at 100%. A test case for every new line of code.
• If applicable, the documentation is updated.
• The source code is amalgamated by running make amalgamate.

Read the Contribution Guidelines for detailed information.

[github-actions]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @trdesilva
Please read and follow the Contribution Guidelines.