[Feature request] Multi-answer challenge

## Description

Today it is possible to add multiple flags to a challenge but all the flags are alternatives. Any one of them is enough to validate the challenge.

Add the possibility to make several flags required to validate the challenge.

For example you add 9 flags to a challenge but you don't want that any of the 9 flags validate the chall you want that the player need to find all the 9 flags to validate the challenge.


## Implementation

- Add a `Required` option to flag creation (more flexible)
- **OR** add a `All the flags are required to validate the challenge` option on the Challenge creation (flag tab) (less flexible but easier to use).

Start by tracing the existing flag-creation and challenge-validation flows described in the issue, including the challenge creation flag tab. Compare the two proposed UI designs and identify how required flags are represented and checked. Done means a challenge can require all selected flags, with coverage for both creation and validation behavior.

Written by the indexing model from the issue text.