Suggestion: Separate explorer GraphQL API endpoint

[moreal]

Purpose

This pull request suggests to separate explorer GraphQL API endpoint from current endpoint /graphql. That means it separates schemas by using purpose. StandaloneSchema and ExplorerSchema. And they will be assigned to /graphql and /explorer/graphql.

What will be better than now?

In 9c-headless side, it can avoid naming conflicts. Further more, the libplanet-explorer-frontend can recover generic feature. Originally, it had targeted to Libplanet.Explorer but now it is targeting 9c-headless because of planetarium's situation. So other Libplanet.Explorer users cannot use the frontend. If 9c-headless separates it and standardize to use LibplanetExplorerSchema, the frontend can be used for more generic projects.

[moreal]

This discussion's suggestion was implemented in #1706.