Define a GraphQL schema

Define GraphQL schema for the current data model. The idea is to provide all the data available in v1 API.

related to swh-web#3951

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Migrated from T3932 (view on Phabricator)

mentioned in merge request !3 (closed)

assigned to @jayeshv

marked this issue as related to #3405 (closed)

added GraphQL API priority:Normal labels

marked this issue as related to #4083

added state:wip label

mentioned in commit 2736e5f3

removed state:wip label

closed