Support using SSL with PEM strings

[stephen37]

Describe the bug

It is not possible to add CA certificate by PEM string when using SSL with Kafka when using the configuration ssl.ca.pem. This is because it has been added in the version v1.8.2 of librdkafka which has been released in October 2021.
The executor is using the version v1.4.2 which has been released in May 2020.

To reproduce

Try to define ssl.ca.pem in the ConfigMap used by Kafka, e.g.

kafkaConfig: kafka.ConfigMap{
    "security.protocol":   "SSL",
    "ssl.ca.pem":          caPEM,
    "ssl.key.pem":         keyPEM,
    "ssl.certificate.pem": certPEM,
    "ssl.key.password":    keyPassword,
},

it will result in an error saying "No such configuration property: "ssl.ca.pem\",. You can make it crash easily by changing the worker_test.go configuration https://github.com/SeldonIO/seldon-core/blob/master/executor/logger/worker_test.go#L140-L146

Expected behaviour

It should be possible to define that option in the configuration, it is defined in librdkafka which is what confluent-kafka-go is using behind the scene.

We have to wait for this issue to be addressed until we can make that possible: #3866

PR to solve this issue: #3868