feat: add LLM assessment for free-forms and emails

### Additions

* added `LLMAssessmentType` type
* added field `llmAssessment: LLMAssessmentType` on `EmailAddressType`
* added field `llmAssessment: LLMAssessmentType` on `FreeFormQuestionDetailsType`
* added field `llm: Boolean!` on `ExerciseType` (indicates, whether the llm features should be shown and used in the exercise)
* added field `llm: Boolean! = false` on `CreateExerciseInput`
* added query `freeFormAssessment(questionnaireSubmissionId: ID!, questionId: ID!): String!`
* added query `emailAssessment(actionLogId: ID!, assessmentId: ID!): String!`

Closes #544