xds: fix the validation code to accept new-style CertificateProviderPluginInstance wherever used (v1.44.x backport)#8901

sanjaypujare merged 1 commit intogrpc:v1.44.xgrpc/grpc-java:v1.44.xfrom

sanjaypujare:1.44.x.security-fixsanjaypujare/grpc-java:1.44.x.security-fixCopy head branch name to clipboard