add Dgraph's authentication token

[MichelDiz]

Add support for Dgraph's authentication token.

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This change is 

[paulftw]

Hey @MichelDiz please take a look at cf306a5

I've merged it yesterday as part of adding support for Slash.
I think it allows adding auth tokens to /alter via options.
And also does a couple more things differently: allows users to specify arbitrary headers, allows adding headers to /query and /mutate as well as /alter (that matters for Slash).

In Ratel I do

    const stub = new dgraph.DgraphClientStub(
        url,
        {
            jsonParser: JSONbigint.parse.bind(JSONbigint),
        },
        {
            headers: {
                "X-Auth-Token": "wrK9gCu1fyARDnaZuxR/zvdewK8oiphXpMb6mTxfaW0=",
            },
        },
    );

and then all calls have token inserted.

That one doesn't have documentation though. and could benefit from an explicit setter -- stub.setAuthToken() or something

[MichelDiz]

@danielmai can you confirm if this PR and the commit from Paul is the same thing?

[paulftw]

Closing this one for now, since there are other ways to add custom headers to a stub

[MichelDiz]

@paulftw and what are they? there are docs about it? If I had found anything I wouldn't even start this PR.