Add subject type 'scope' for role mapping (#1014)

[jquirymateco]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)
For OAuth rbac role mapping, allow Spring scopes to be used by using provider type scope. This is useful if no groups are present in the ID token.
The roles-field is now optional.

rbac:
  roles:
    - name: "Admin"
      clusters:
        - non-prod
      subjects:
        - provider: oauth
          type: scope
          value: messages:all

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• I have performed a self-review of my own code
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• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

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[Haarolean]

#1014 (comment)