Add config visibility to the frontend

[grantila]

SonarQube config visible in the frontend

We need to get the urls to the instances in the frontend, but they're not visible, despite being @visibility frontend in the config - although the backend config. I saw that there's a shadow config in the frontend package for the baseUrl, but this only works if you don't have multiple instances.

This PR adds the config values of the baseUrl (and other non-sensitive fields) to the frontend for multi-instance configurations.

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-sonarqube	workspaces/sonarqube/plugins/sonarqube	patch	v0.8.6