Add cluster extension (#6118)
Add cluster extension (#6118)

Signed-off-by: Henry Yang <hyang@lyft.com>

Mirrored from https://github.com/envoyproxy/envoy @ 95998b4cdb017505a3869f892cde25655377f2a8

lyft/envoy-api,lyft/envoy-api,lyft/envoy-api