feat: add LLM capabilities with AI email response suggestions

### Additions

* added query `emailTemplateSuggestion(threadId: ID!, participantId: ID): EmailTemplateSuggestionType!`
  * participantId can be provided in case of multiple definition addresses within the thread the instructor is responding to
  * (in other words, the instructors can choose the definition entity that they want to reply as; otherwise, the LLM will try to pick the best one with the best content)
* added type `EmailTemplateSuggestionType`

Closes https://gitlab.fi.muni.cz/inject/inject-issues/-/issues/55