Sync changes with upstream

Fixes GRAPHQL-789
This PR allows coercing of single values in variables to list.

fix(GraphQL): This PR change the coerced slice to type interface.

The following query was giving panic because here we passed ID as an int which is expected to be a string.

query allStories {
      queryUser(filter: {
        id: 22
      }) {
        stories {
          id
          text
        }
      }
    }

We now added input coercion so that the ID type value will be coerced to string type. And if we give a slice of integer values or the required type is [ID] then that will be coerced to slice of integer values.

fix(GraphQL): Fix bug that got introduced in last commit , which doesn't allow string value in [ID] to be coerced to list.

Fixes GRAPHQL-1006
In this PR we have added input coercion for Integers which are passed in variables.
Previously this code was part of dgraph.

…bles

Fixes: GRAPHQL-1007
We have allowed to paas @cascade arguments through variables in dgraph. For that we needed input validations check which is added in this library.

fix(GraphQL): fix validation when we give non-string value in variable and expected type is string.

fix(GraphQL): fix error while validation when we give Int or Float to String.

fix(GRAPHQL): Remove support of String --> int32 coercion in variables.

fix(Parser): Add directive argument validator

chore: adding order to the validation rules

fix(validate): fix race condition in rules sort

Signed-off-by: Steve Coffman <[email protected]>