chore(prdsub): Add external secret for clickhouse-cloud-dsn to DIP release

What

• Defines external secret for clickhouse-cloud-dsn populated from vault in DIP release for prdsub
• Also adds config for billing exporter to prdsub

Why

• Enable billing exporter and connect with clickhouse cloud to enable flow of usage events.

Related to https://gitlab.com/gitlab-org/analytics-section/platform-insights/core/-/issues/96 and https://gitlab.com/groups/gitlab-org/analytics-section/-/epics/14