feat(acl): allow access to all the predicates using wildcard

[NamanJain8]

There are usecases that need read/write/modify permissions over all the predicates of the namespace. It is quite tedious to manage the permissions every time a new predicate is created.
This PR adds a feature to allow a group, access to all the predicates in the namespace using wildcard dgraph.all.

This example provides to dev group, read+write access to all the predicates

mutation {
  updateGroup(
    input: {
      filter: { name: { eq: "dev" } }
      set: { rules: [{ predicate: "dgraph.all", permission: 6 }] }
    }
  ) {
    group {
      name
      rules {
        permission
        predicate
      }
    }
  }
}

NOTE: The permission to a predicate for a group (say dev) is a union of permissions from dgraph.all and the permissions to specific predicate (say name). So suppose dgraph.all is given READ permission, while predicate name is given WRITE permission. Then the group will have both READ and WRITE permission.

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This change is