Optimise process_entity_members performance

## Describe the bug
the `process_entity_members` in `owasp_update_leaders.py` has 2 nested loops , running in O(m*n) time, which doesn't scale well. I am confident it can be optimized to O(m+n) by implementing a look up map, increasing performance.

### To Reproduce
In the `process_entity_members` function we iterate through each leader (N). Inside this loop, we call
`find_best_user_match` which iterates through the entire list of GitHub users (M) to find a match
hence O(M*N)

### Are you going to work on fixing this?

- [x] Yes
- [ ] No

### screenshots
- NA