[MM] NestedObject flatten

[ArcturusZhang]

If a NestedObject only contains one field, the reviewers from terraform would require us to flatten the structure. For example, on behalf of cli, we have an api.yml saying

      - !ruby/object:Api::Type::NestedObject
        name: 'identity'
        description: ''
        required: false
        azure_sdk_references: ['/identity']
        properties:
          - !ruby/object:Api::Type::Enum
            name: 'type'
            description: 'The type of Managed Identity used by the DiskEncryptionSet. Only SystemAssigned is supported.'
            required: false
            azure_sdk_references: ['/identity/type']
            values:
              - :SystemAssigned
            default_value: :SystemAssigned

This should be replaced by

      - !ruby/object:Api::Type::Enum
        name: 'identityType'
        description: 'The type of Managed Identity used by the DiskEncryptionSet. Only SystemAssigned is supported.'
        required: false
        azure_sdk_references: ['/identity/type']
        values:
          - :SystemAssigned
        default_value: :SystemAssigned

This issue was initially filed to MM here

[zikalino]

this can be fixed already.
if you add:
flatten-all: true
to readme.cli.md file it will flatten everything by default