Allow to update application.properties for pulsar-manage

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

`pulsar-manager` pod comes with the following `application.properties`:
```
...
# If user.management.enable is true, the following account and password will no longer be valid.
pulsar-manager.account=pulsar
pulsar-manager.password=pulsar
# If true, the database is used for user management
user.management.enable=true
...
```

The last option invalidates the `pulsar`/`pulsar` login.

**Describe the solution you'd like**

I'd like to configure the access credentials via a standard Kubernetes `secret`, but since pod doesn't have a way to update individual values or whole `application.properties` it's not working.

Ideally, it should be able to read `application.properties` from a ConfigMap where I can disable/enable user management, and set default values, as well as placeholders for Spring.

Currently, the Deployment already reads the `USERNAME` and `PASSWORD` from a secret, but because it's disabled in `application.properties` it's not helping.

Start by inspecting the chart's Deployment and its existing USERNAME/PASSWORD Secret wiring. Define how a ConfigMap-provided application.properties should be consumed alongside Secret credentials, then verify the rendered Deployment supports enabling user management and configuring the default values.

Written by the indexing model from the issue text.