Provisioner - enable oidc extension by default for newly provisioned clusters

[koala7659]

Enabling OIDC extension for Provisioned shoots by adding following item to list of shoot extensions

extensions:
   - type: shoot-oidc-service  #this line enables OIDC extension
     disabled: false

This change will affect only new clusters as specified in #3466

Extension section from test shoot provisioned on AWS

extensions:
  - type: shoot-dns-service
    providerConfig:
      apiVersion: some-version
      dnsProviderReplication:
        enabled: true
      kind: DNSConfig
      providers:
        - domains:
            include:
              - some-domain
          secretName: some-name
          type: some-type
      syncProvidersFromShootSpecDNS: true
  - type: shoot-cert-service
    providerConfig:
      apiVersion: some-api-version
      shootIssuers:
        enabled: true
      kind: CertConfig
  - type: shoot-networking-filter
    disabled: true
  - type: shoot-oidc-service
    disabled: false
  - type: shoot-auditlog-service
    providerConfig:
      kind: AuditlogConfig
      apiVersion: some-api-version
      type: standard
      tenantID: some-tenant
      serviceURL: some-url
      secretReferenceName: some-name

[m00g3n]

LGTM