Adds callback to obtain SASL PLAIN credentials

[josvisser66]

This PR adds a callback for obtaining the SASL PLAIN username and password. The use case here is to support changing the credentials to be used for a Kafka cluster without bouncing the Kafka client (e.g. in the case of credential rotating).

The user can set a credentials callback using the new API function rd_kafka_conf_set_plain_creds_cb. That callback will be called by the SASL implementation whenever it needs a SASL username, password, or both. The SASL provider does not cache the provided credentials as this would defeat the purpose.

Trivial and not terribly correct example:

int sasl_creds_cb(rd_kafka_t *rk,
                  char *username,
                  int username_size,
                  char *password,
                  int password_size) {
    if (username) {
        strcpy(username, kUsername);
    }
    if (password) {
        strcpy(password, kPassword);
    }
    return 0;
}

...
    conf = rd_kafka_conf_new();
    /* Do not set sasl.username and sasl.password */
    rd_kafka_conf_set_plain_creds_cb(conf, sasl_creds_cb);