Allow for sourcing of Fluent Bit Kafka Rdkafka values from secrets

### 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.