feat: add simple KV store

### Additions

* added new query `readValue(exerciseId: ID!, key: String!): String!`
* added new type `KeyValueType`
* added new query `exerciseValues(exerciseId: ID!): [KeyValueType!]!`
* added new mutation `writeValue(exerciseId: ID!, key: String!, value: String!): String!`
* added new mutation `clearValue(exerciseId: ID!, key: String!): Boolean!`
* added new subscription `kvStore(exerciseId: ID!): (key: String!, value: String)`

Closes #291