Add a couple features to work towards feature parity with old binderhub chart

[yuvipanda]

• allow setting extraVolumes and extraVolumeMounts on the binder pod
• Don't override enable_api_only_mode - use whatever the default in binderhub is

The eventual goal here is to replace https://github.com/jupyterhub/binderhub/tree/main/helm-chart with this project - see jupyterhub/binderhub#1862.