feat+fix(lambda): allow access to access jwt in lambda

[NamanJain8]

Feature:
This change makes X-Dgraph-AccessToken accessible to the lambda via accessToken argument. This can help users to extract the User ID from there and taking the relevant actions.
Example script:

function parseJwt (token) {
    var base64Url = token.split('.')[1];
    var base64 = base64Url.replace(/-/g, '+').replace(/_/g, '/');
    var jsonPayload = decodeURIComponent(atob(base64).split('').map(function(c) {
        return '%' + ('00' + c.charCodeAt(0).toString(16)).slice(-2);
    }).join(''));

    return JSON.parse(jsonPayload);
};

async function newMovie({args, accessToken, graphql}) {
	console.log(parseJwt(accessToken))
        // Output: {"exp":1631636949,"groups":["guardians"],"namespace":0,"userid":"groot"}
        // Do work.
        return result.data.addMovie.movie[0]
}

self.addGraphQLResolvers ({
    "Mutation.newMovie": newMovie
})

Fix:
Earlier, we used statically generated env.AUTH_TOKEN to pass the credentials from Dgraph to lambda. This would not work with the current implementation of lambda. Here, we need to pass X-Dgraph-AccessToken.

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This change is