Implement serviceAccount support for clickhouse pod

[laurencegill]

Description

In order to use IRSA for clickhouse-backup I require the ability to use a custom serviceAccountName and annotations for the clickhouse pod. This change adds the parameters to configured that from within the chart:

clickhouse:
  serviceAccount:
    create: true
    name: "my-service-account"
    annotations:
      eks.amazonaws.com/role-arn: "arn:aws:iam::111122223333:my-role/my-service-account"

Alternatively the SA can be configured externally and added:

clickhouse:
  serviceAccountName: "my-service-account"

#463 seems related (not my issue)

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How has this been tested?

This change is deployed in a staging environment using a fork of the posthog repo

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works