Allow to update application.properties for pulsar-manage

[splix]

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.