[FEATURE]: pass JWT token to GraphQL Lambda

[rderbier]

We need a way to make X-Dgraph-AccessToken accessible to the lambda so that logic can extract the UserID.

We have customers requesting this: https://dgraphlabs.zendesk.com/agent/tickets/1869

Done in v21.12 in #8026 and 8023.
We had a previous PR: #8023 which works with the re-architected lambda implementation introduces in 21.12.

[rderbier]

Create a cherry-pick

[rderbier]

See Florian's test case with tenants at https://discuss.dgraph.io/t/x-dgraph-accesstoken-does-not-get-extracted-in-lambda/18465

[rderbier]

AccessJWT token is passed by Alpha.
Close this issue and replace by
https://github.com/dgraph-io/dgraph-lambda/issues/42
as the problem is that the access token is not used in lambda.