feat(GraphQL): @custom HTTP body now supports hardcoded scalars

[abhimanyusinghgaur]

Fixes #5641
Fixes GRAPHQL-508
Fixes Discuss Issue

This PR adds support for parsing hardcoded scalar values in @custom HTTP body.
This also adds support for allowing _ in key names in the body template.

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This change is 

[pawanrawal]

Reviewed 7 of 7 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @abhimanyusinghgaur)

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graphql/schema/wrappers.go, line 1866 at r1 (raw file):

// { "author" : "$id", "post": { "id": "$postID" }} and { "id": true, "postID": true}
// If the final result is not a valid JSON, then an error is returned.
// strictJSON should be false when the body template is being used for custom graphql arg parsing,

In strictJSON mode block strings and enums are invalid and should throw an error.

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graphql/schema/wrappers_test.go, line 610 at r1 (raw file):

			nil,
			nil,
			errors.New("input:1: Unexpected ("),

Can we send this as part of the full error when a schema update happens?

[abhimanyusinghgaur]

Reviewable status: 4 of 7 files reviewed, 2 unresolved discussions (waiting on @MichaelJCompton and @pawanrawal)

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graphql/schema/wrappers.go, line 1866 at r1 (raw file):

Previously, pawanrawal (Pawan Rawal) wrote…

In strictJSON mode block strings and enums are invalid and should throw an error.

Done.

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graphql/schema/wrappers_test.go, line 610 at r1 (raw file):

Previously, pawanrawal (Pawan Rawal) wrote…

Can we send this as part of the full error when a schema update happens?

Done.