Challenge Search Api, events param validation error.

Steps to Reproduce:

1. Run curl https://api.topcoder-dev.com/v5/challenges/\?events\[\]\=tco20
2. See error: "{"message":""0" must be a number"}"

Expected:

• Api should return challenges related "TCO20" events.

Problem:

https://github.com/topcoder-platform/challenge-api/blob/develop/src/services/ChallengeService.js#L675

• events validation should be Joi.array().items(Joi.string()) instead of number.

Final Submission Guidelines

Git issue link: https://github.com/topcoder-platform/challenge-api/issues/346