Upgrade dgrijalva/jwt-go/v3 to v4 to fix Mend report vulnerabilities

[shawnhankim]

Background

• NIST CVE-2020-26160
  • jwt-go before 4.0.0-preview1 allows attackers to bypass intended access restrictions in situations with []string{} for m["aud"] (which is allowed by the specification).
  • Because the type assertion fails, "" is the value of aud.
  • This is a security problem if the JWT token is presented to a service that lacks its own audience check.
• Related MR for dgrijalva/jwt-go: https://github.com/dgrijalva/jwt-go/pull/426

Fix

• [x] Add go.mod
• [x] Upgrade dgrijalva/jwt-go/v3 to v4 to fix Mend report vulnerabilities
• [x] Fix fields' format of jwt.StandardClaims after upgrading dgrijalva/jwt-go