add ca to prevent middleman

[DBC201]

/// The security context for secure usage
SecurityContext securityContext = SecurityContext.defaultContext;

/// Callback function to handle bad certificate. if true, ignore the error.
bool Function(X509Certificate certificate)? onBadCertificate;

• define these in mqtt provider to prevent man in the middle attacks. security context should be the ca certificate given by let's encrypt. this is alerady added in config/ssl.dart

• the callback function should be to what to do when the validation fails

[DBC201]

this function is predefined, no need to define again