add Bigbird ONNX config
# What does this PR do?

Add Bigbird OnnxConfig to make this model available for conversion.

## Who can review?
@lewtun @LysandreJik
_The documentation is not available anymore as the PR was closed or merged._<|||||>5040f17eba15504bad66b14a645bddd9b015ebb7 #15622 <|||||>@lewtun Thank you for reviewing my code. I have fixed the merge conflicts and all tests by running this `RUN_SLOW=1 pytest tests/onnx/test_onnx_v2.py -k "bigbird"` was passed.<|||||>This all looks good to me @vumichien - thanks for iterating! Gently pinging @LysandreJik or @sgugger for final approval