Skipper: opaAuthorizeRequestWithBody filter bypasses OPA policy on Transfer-Encoding: chunked / HTTP/2 requests

Skipper is an HTTP router and reverse proxy for service composition. Prior to 0.26.10, zalando/skipper's OpenPolicyAgent integration silently bypasses request-body inspection on HTTP/1.1 Transfer-Encoding: chunked and HTTP/2 requests that omit the content-length pseudo-header, because the opaAuthorizeRequestWithBody filter and OpenPolicyAgentInstance.ExtractHttpBodyOptionally in filters/openpolicyagent/openpolicyagent.go produce an empty raw_body and input.parsed_body while the upstream service receives the full attacker-controlled body. This issue is fixed in version 0.26.10.