feat(GraphQL): Support using custom DQL with @groupby

[minhaj-shakeel]

Fixes GRAPHQL-754.
This PR adds support for the @remoteResponse directive in the GraphQL schema. Users can define this directive on the fields of a @Remote type in order to map the response JSON key of a custom query to a GraphQL field.
For example, for the given GraphQL schema:-

type User {
	screen_name: String! @id
	followers: Int @search
	tweets: [Tweets] @hasInverse(field: user)
}
type UserTweetCount @remote {
	screen_name: String
	tweetCount: Int
}
type UserMap @remote {
	followers: Int
	count: Int
}
type GroupUserMapQ @remote {
	groupby: [UserMap] @remoteResponse(name: "@groupby")
}

the below @Custom DQL query is made possible:-

queryUserKeyMap: [GroupUserMapQ] @custom(dql: """
{
	queryUserKeyMap(func: type(User)) @groupby(followers: User.followers){
		count(uid)
	}
}
""")

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This change is 

[abhimanyusinghgaur]

Reviewed 4 of 4 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @minhaj-shakeel and @pawanrawal)

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graphql/schema/rules.go, line 2122 at r1 (raw file):

"Type %s; Argument %s inside @remoteResponse directive must be defined."

also mention the field in the error message

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graphql/schema/wrappers.go, line 923 at r1 (raw file):

	fd := f.field.ObjectDefinition.Fields.ForName(f.Name())
	remoteResponseName := remoteResponseDirectiveArgument(fd)

The ForName() lookups here may turn out to be expensive.
Let's store the remoteResponse names in our schema wrapper like we store dgraphPredicate, customDirectives, lambdaDirectives mapping there and then remove the directive from the field definition once it has been added to the mapping.

[abhimanyusinghgaur]

Reviewed 59 of 59 files at r2, 2 of 2 files at r3.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @minhaj-shakeel and @pawanrawal)