At AegisCloud, certificate identifiers must follow a strict string format before they can be processed. Implement a function that returns True if a certificate string is valid and False otherwise.
A certificate string is valid if all of the following are true:
header:payload:signature.A-Z), digits (0-9), and the delimiter characters (, ), [, ], {, }.cert, a stringbool — whether the certificate format is validdef validate_certificate(cert):