feat: allow overriding the update interval when creating exercise

### Additions

* added new type `ConfigType`
* added new enum `Feature`
* added field `config: ConfigType!` to `ExerciseType` type
* added field `config: ConfigType!` to `DefinitionType` type
* added field `updateIntervalS: Int` to `ConfigOverrideInput` type
* added field `updateIntervalS: Int` to `ConfigOverrideType` type

### Deletions

* removed type `GrapheneConfig`
* removed query `exerciseConfig`

Closes #370