Allow for sourcing of Fluent Bit Kafka Rdkafka values from secrets

[jeff303]

Is your feature request related to a problem? Please describe.

Currently, the Kafka struct declares the Rdkafka map as map[string]string, which means that values must be stored in plaintext in the CRD. However, when connecting to a Kafka broker using SASL_SSL (i.e. when there is a username/password corresponding to the API key/secret, respectively), then it would be better to not store those values in plaintext.

Describe the solution you'd like

Modify the Rdkafka map to allow for values to be either string or *plugins.Secret. That way, users can designate a secret ref for particular values (such as sasl.username and sasl.password), so they don't need to be provided in plaintext at the CRD level.

Additional context

This is useful in multi-tenant scenarios.