Add additional configuration to externalKafka

[logand22]

Proposed change

I'd like the ability to configure Kafka security protocol as well as SASL parameters via the externalKafka helm chart values.

For example:

# Ignored if `kafka.enabled` is set to `true`. 
externalKafka:
  # - External Kafka brokers. Multiple brokers can be provided as array/list.
  brokers: []
  # - External Kafka security protocol.
  securityProtocol: ""
  # - External Kafka authentication SASL Mechanism.
  saslMechanism: ""
  # - External Kafka SASL user.
  saslUser: ""
  # - External Kafka SASL password. Either this or `externalKafka.existingSecret` must be set.
  saslPassword: ""
  # - Name of an existing Kubernetes secret object containing the Kafka SASL password.
  existingSecret: ""
  # - Name of the key pointing to the password in your Kubernetes secret
  existingSecretPasswordKey: "password"

Alternative options

You can add the environment variables as defined in PostHog data sources directly through the env configuration section in .values.yaml.

This doesn't allow the user to specify a secret for the password or the ClickhouseInstallation.

Additional context

I'm deploying PostHog with Amazon MSK and would like to use SASL/SCRAM authentication but don't want to have password exposed in helm chart values. Additionally, I had to look into PostHog source code to understand how to configure this. Having these values in the helm chart could improve the UX for this chart.