fix(graphql): override nodetype tostrings for display

[andrewazores]

Fixes #853

I found this simple workaround for the way that GraphQL treats enum values. Rather than creating some duplication within our internal GraphQL module that re-encodes all of the NodeType enums to their Kind strings, I simply add an override on the toString() method for all of the NodeType implementations that simply returns the Kind. GraphQL uses toString() to serialize the nodeType enum field, which displays the enum member's name by default, so this override is simple and effective and doesn't duplicate definitions.

To test:
Query http :8181/api/beta/graphql query=@graphql/root-node-query.graphql
Query http :8181/api/v2.1/discovery

Compare the results of the kind/nodeType fields of nodes, before and after this patch. Before you should see ex. REALM/Realm, whereas after they will both agree on Realm.

[hareetd]

Looks good!