ExternalObjectRequest compression

[abrown-RV]

on the REST side we have a way for the client to request a compression method in the response. This is currently not possible with gRPC

Suggest adding PayloadEncoding payload_encoding = <> to ExternalObjectRequestPayload May also need to pull enum PayloadEncoding out into the global scope.

openapi.yaml has this for the asset request

    - name: compression
      in: query
      required: false
      description: The compression method to use. If not provided, the asset will be sent uncompressed.
      schema:
        type: string
        enum: [UNCOMPRESSED, GZIP, DEFLATE]
      example: "UNCOMPRESSED"

[mejohnnaylor]

discovered we need to document ExternalObjectRequestPayload in -19