Add a matrix bot for answering questions

A matrix should be implement that supports question answering based on an LLM configured in the LLM Application.

It should allow answering a question that is in a single message to the bot. If there are several users in a room, the bot should only react to messages explicitly mentioning it. The bot should also support commands to select the model and to list all available model.

In terms of permissions, it should be possible to configure which users may use the bot (invite the bot into a room or start a chat with it). Further, it should be possible to configure a username/password combination that is used by the bot to authenticate in XWiki. It should be possible to override this user information for individual rooms or users (supporting some form of wildcards or regular expressions to match several rooms/users) such that some rooms could use a different user account to, e.g., get access to different context information or different models (e.g., a fast model with public context for public rooms and better but more expensive models with more context for private rooms).