Handle pickling top-level enums.

[claudevdm]

#21298

This handles pickling of enums that are defined at the top level

A top level enum is defined like

enum Enum {
  UNSPECIFIED = 0;

  // Event time is time from the perspective of the data
  EVENT_TIME = 1;
}

A nested enum is defined like
message TimeDomain {
  enum Enum {
    UNSPECIFIED = 0;

    // Event time is time from the perspective of the data
    EVENT_TIME = 1;

  }
}

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[claudevdm]

R: @tvalentyn

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[claudevdm]

I could not find any top level proto enums to use for writing a unit test, let me know if I should create some fake protos for test coverage?

[tvalentyn]

Do we have a test that exercises this?. nvm, just saw the comment above

[claudevdm]

Do we have a test that exercises this?. nvm, just saw the comment above

Just added tests and regenerated the proto