Add encoding interfaces for DER encoding:
1. support decoding of 'bit string', 'octet string', 'object id'
and 'context specific tag' for DER encoder.
2. implemented a simple DER encoder.
3. add more testsuits for DER encoder.

Signed-off-by: lei he <helei.sig11@bytedance.com>
Message-Id: <20221008085030.70212-3-helei.sig11@bytedance.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>