[FEATURE]: Add multi-league support for contests

Problem / opportunity:

Currently, contests are added to contests.csv with a single value to indicate what chain they're on: EVM or Cosmos. But some past contests have told us that they're really both (EVM and Cosmos). This seems likely to become more of an issue as we add more chains.

Sponsors have asked if we can note on the website that their contest falls into both categories, but we haven’t had a way to do this.

Desired outcome

It would be preferable if we could:

• list multiple chains where appropriate;
• consider how to display contests that are "tagged" with multiple chains (e.g. on https://code4rena.com/contests/)
• integrate this into any UX improvements we make to our Reports section.

Solution

Add multi-league support to contests.csv so the C4 website can display them on the appropriate page/contest list.

Start by inspecting contests.csv and the contest list at code4rena.com/contests/ to understand the current league value and display flow. Review the linked Reports issue (#2116) for related UX direction. Done means contests can carry multiple chains and appear appropriately in contest lists and the Reports experience.