Through importing but not using rune/libenclave/attestation in gen-quote.go
to make the rune/libenclave/attestation package under vendor.
Signed-off-by: NYilin Li <YiLin.Li@linux.alibaba.com>

The new implementation introduces new object called Challenger
and the foundations for SGX EPID-based RA. In future, the object
called Attester will be added to complete the attestation package.
Signed-off-by: Jia Zhang <zhang.jia@linux.alibaba.com>