Add separate handler to create grand central resources

[tomach]

Summary of changes

Moved the creation of grand central resources from cluster creation process to a separate handler, because it actually has no dependencies on other handlers. This allows installing grand central also for existing clusters by adding a section like this to the CRD:

  grandCentral:
    backendImage: cloud.registry.cr8.net/crate/grand-central:latest
    backendEnabled: true
    apiUrl: https://console.cratedb-dev.cloud/
    jwkUrl: https://console.cratedb-dev.cloud/api/v2/meta/jwk/

Checklist

• [x] Link to issue this PR refers to: https://github.com/crate/cloud/issues/1662
• [x] Relevant changes are reflected in CHANGES.rst
• [x] Added or changed code is covered by tests
• [x] Documentation has been updated if necessary
• [x] Changed code does not contain any breaking changes (or this is a major version change)