rune/libenclave: add remote attestation type related constants.

Signed-off-by: Yilin Li YiLin.Li@linux.alibaba.com

rune/libenclave/attestation/sgx/attest.go

+package sgx // import "github.com/opencontainers/runc/libenclave/attestation/sgx"
+
+// RA Type
+const (
+	InvalidRaType = iota
+	EPID
+	DCAP
+)