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

While it is possible to use an external PostgreSQL database, the Helm Chart provides no support for deploying and configuring one within a Kubernetes cluster, as it does for Apache Cassandra. This issue requests that support to be added (similar to what is requested for MySQL in #635).

Describe the solution you'd like

A cluster deployed through the Helm Chart would continue to use Cassandra and Elasticsearch by default, but one could optionally set a property postgresql.enabled to true to deploy and use a PostgreSQL container for both persistence and visibility instead. There may be additional properties for customizing aspects, such as the username and password, for that PostgreSQL deployment.

This issue does not specifically request the ability to use PostgreSQL for persistence and another SQL system (such as MySQL) for visibility. The only expected supported combinations are PostgreSQL for both, and PostgreSQL for persistence with Elasticsearch for visibility. Support for other combinations may be added in the future, but need not be addressed for the initial implementation described here.

Helm best practises should be followed so that choosing to use a sub-chart or an external system are done in a predictable and clear way. Where possible we should avoid the need to duplicate values, the system should infer the Postgres hostname from the sub-chart, and so on.

Additional context

The PostgreSQL version deployed should be the latest stable version supported by the Temporal Server software being deployed. In the case of known incompatibilities with that version of PostgreSQL, it might deploy a different version.