[Feature Request] Add support for deploying and using MySQL

[tomwheeler]

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

While it is possible to use an external MySQL 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.

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 mysql.enabled to true to deploy and use a MySQL container for both persistence and visibility instead. There may be additional properties for customizing aspects, such as the username and password, for that MySQL deployment.

This issue does not specifically request the ability to use MySQL for persistence and another system (such as Elasticsearch) for visibility. Support for such combinations may be added in the future, but need not be addressed for the initial implementation described here.

Additional context

The MySQL 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 MySQL, it might deploy a different version.