rbac-backend plugin: implement loading conditions from file

[AndrienkoAleksandr]

Provide a clear and concise description of the feature request

rbac-backend plugin: implement loading backstage conditions from file.

Describe the problem that this feature resolves

The RBAC-backend plugin should include support for backstage conditions and offer a REST API for this feature. Additionally, it would be beneficial to allow the creation of conditions using an external file. The external file path can be configured through the application context for flexibility.

Additional context or screenshots for the feature request

Acceptance Criteria

User can provide pre-defined conditions for backstage/showcase instance with help of external file.

[AndrienkoAleksandr]

Currently, we have a feature that allows users to define RBAC policies or group policies using an external file. The RBAC-backend plugin uses the CSV format, where each new policy or group policy is defined on a single line. However, backstage conditions can be quite extensive, because condition content is in JSON format.

To simplify the maintenance of these conditions, I propose setting up conditions using separate JSON or YAML file.