[Breaking] fix(GRAPHQL): fix input coercing for integers and make them GraphQL spec complaint.

[JatinDev543]

GraphQL spec doesn't allow any value apart from integer to be passed to a variable of type Int. So we are disallowing strings to be passed as Int variables. Int64 is our own scalar data type, so we continue to accept string and Integer values for it.
This PR allows passing strings to Int64 values when given directly in query/mutations and disallows string to be passed to Int in variables.

So we will have below final behavior:

`Int32` will accept only `Integer` values, and otherwise will give a coercion error.
`Int64` will accept `Integer` as well as `String` value and will return an error if any other value. And if we can only pass Integer in a string like "1234" otherwise it will give a coercion error.

Int coercion rules in GraphQL spec:
https://spec.graphql.org/June2018/#sec-Int

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This change is 

[abhimanyusinghgaur]

Nice change in test.

In the PR title and description, we should correct some typos:

• Graphql -> GraphQL
• specs -> spec
• apart from Int to be -> apart from integer to be (basically, Int anythwere -> integer)
• variable of type Int32 -> variable of type Int (basically, Int32 anywhere -> Int)

Reviewed 4 of 4 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @manishrjain, @pawanrawal, and @vvbalaji-dgraph)