Module window_frame

Nested message and enum types in WindowFrame.

Enums

• EndBound
  “optional” keyword is stable in protoc 3.15 but prost is still on 3.14 (see https://github.com/tokio-rs/prost/issues/430 and https://github.com/tokio-rs/prost/pull/455) this syntax is ugly but is binary compatible with the “optional” keyword (see https://stackoverflow.com/questions/42622015/how-to-define-an-optional-field-in-protobuf-3)