Allow option to load MYSQL_PASS from file instead of ENV

[garygan89]

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

No. But having the option to load password from file would allow easy integration with external centralized secret storage such as Hashicorp Vault. This is great for operator that do not want to store credentials in k8s etcd as Secret (which is unencrypted by default), and rely on external vault service.

The Vault Agent Injector can automatically mount the secret as a file to the kubevious pod, which the path can then sent to the arguments.

Describe the solution you'd like

Allow passing argument such as the following to load from file

--mysql-password-from-file=/vault/secrets/mysql-pass

If the file is not specified, then fallback to ENV or string literal --mysql-password-literal=myawesomepassword

Describe alternatives you've considered

Patch the Deployment with Hashicorp-vault-agent-injector annotations to mount, then template it to ENV.

Additional context

None