[gRPC] Support integral enumerations #629

Protobuf supports integral enums using the enum keyword. We should be able to map Cadl enum to Proto3 enum:

@service interface WidgetService {
  paint(@field(1) id: int32, @field(2) color: WidgetColor): void;
}

enum WidgetColor {
  RED: 0,
  BLUE: 1
}

This should yield the following service:

enum WidgetColor {
  RED = 0;
  BLUE = 1;
}

message PaintRequest {
  int32 id = 1;
  WidgetColor color = 2;
}

service WidgetService {
  rpc paint(PaintRequest) returns Empty;
}

There are some considerations:

• The values must be int32 values, and there must be a zero value (which is implied to be the default value), so we would likely require Cadl enums to explicitly specify integral values to use them in gRPC emit.
  • We could also provide a template or decorator to "gRPC-ify" an enum.
• The zero value is required to be the first value in the enum for proto2 compatibility reasons.
• Protobuf enums support an option called allow_alias (off by default) that will permit multiple keys of the enum to refer to the same integral value.