Add slash commands (tasks) for pre-defined tasks

Add support for slash commands to Gitlab Duo Chat:

allows defining one or more slash commands for chain tools
when user sends a command starting with / (e.g. /refactor), then we skip zero shot agent and the tool the command belongs to is called directly. Additional context about selected code os passed to the tool.
slash commands should also accept addition user's text (e.g. /refactor use Rails framework) which should be then added into command's intruction too

Depending if one tool can be reused for multiple commands, we can have either one command per tool or multiple commands per tool (related discussion: &11734 (comment 1621754639)

For MVC, /explain, /refactor and /write_tests commands are planned:

Edited Nov 13, 2023 by Jan Provaznik