Consider adding FrontendAttributesAttr and BackendConfigAttr

[burmako]

Found during the #565 audit in hlo_instruction.cc.

  if (proto.has_frontend_attributes()) {
    instruction->set_frontend_attributes(proto.frontend_attributes());
  }

  // Generic map of attributes used to pass hints / configuration options from
  // the Python frontend to the XLA backend.
  message FrontendAttributes {
    map<string, string> map = 1;
  }

  instruction->backend_config_ = proto.backend_config();

  // Backend configuration for the instruction. Has backend-specific meaning.
  bytes backend_config = 43;

These are some powerful extensibility mechanisms, but it is not immediately obvious that we want extensibility in StableHLO to look like this. Features that allow attaching arbitrary strings to arbitrary ops conflict with StableHLO's focus on crystal clear semantics and strong compatibility guarantees, so this will need a dedicated discussion.