Feat(GraphQL): Add count query feature at root to GraphQL

[vmrajas]

Motivation:
The PR adds

1. aggregateTypeName query and AggregateResultTypeName Type to GraphQL schema. This is used for adding support for Count Queries at root to GraphQL.
2. Adds functionality for count queries at root in GraphQL.
3. Added tests in graphql/resolve/
   Related Discuss Issue: https://discuss.dgraph.io/t/count-queries-in-graphql/10714/9
   This PR will be merged to rajas/count_queries_main which will then be merged to master branch once all features for handling count queries are present.

Fixes GRAPHQL-746

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This change is 

[pawanrawal]

Reviewed 52 of 52 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @MichaelJCompton and @vmrajas)

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graphql/e2e/auth/auth_test.go, line 895 at r1 (raw file):

	for _, tcase := range testCases {
		t.Run(tcase.role+tcase.user, func(t *testing.T) {
			getUserParams := &common.GraphQLParams{

Just params is enough instead of getUserParams

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graphql/e2e/auth/auth_test.go, line 956 at r1 (raw file):

	for _, tcase := range testCases {
		t.Run(tcase.role+tcase.user, func(t *testing.T) {
			getUserParams := &common.GraphQLParams{

params

[vmrajas]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @MichaelJCompton and @pawanrawal)

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graphql/e2e/auth/auth_test.go, line 895 at r1 (raw file):

Previously, pawanrawal (Pawan Rawal) wrote…

Just params is enough instead of getUserParams

Done.

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graphql/e2e/auth/auth_test.go, line 956 at r1 (raw file):

Previously, pawanrawal (Pawan Rawal) wrote…

params

Done.