[Feature]: Make RSS FreeDiskSpacePolicy actually configurable

[fstagni]

User Story

As a admin,
I want to be able to set the thresholds for the FreeDiskSpacePolicy,
so that different VOs can set different ones

Feature Description

The current thresholds are effectively hardcoded (see Configurations.py file). A CS exposure is needed.

Definition of Done

• feature work as expected

Alternatives Considered

No response

Related Issues

No response

Additional Context

No response