Support proxy option for common.RemoteDownload

[peichengATgit]

The gnoi.common.RemoteDownload defines the HTTP/HTTPS options as the following :
gnoi.common

message RemoteDownload {
  // The path information containing where to download the data from or to.
  // For HTTP(S), this will be the URL (i.e. foo.com/file.tbz2).
  // For SFTP and SCP, this will be the address:/path/to/file
  // (i.e. host.foo.com:/bar/baz).
  string path = 1;

  enum Protocol {
    UNKNOWN = 0;
    SFTP = 1;
    HTTP = 2;
    HTTPS = 3;
    SCP = 4;
  }
  Protocol protocol = 2;

  types.Credentials credentials = 3;

  // Optional source address used to initiate connections from the device.
  // It can be either an IPv4 address or an IPv6 address, depending on the
  // connection's destination address.
  string source_address = 4;

  // Optional source vrf used to initiate connections from the device.
  string source_vrf = 5;
}

Though for the HTTP/HTTPS remote paths, there commonly exist cases that the request are only allowed via the proxies, either the forward or reverse proxy.

The current message does not support to express such proxy intent.
Please consider to define the proxy option accordingly.