SPKI doesn't change when certificate is reissued, so it works better
with short-lived certificates.
This is also the same format as used in HTTP Public Key Pinning.
Signed-off-by: Piotr Sikora <piotrsikora@google.com>
As previously pointed out in #9, the empty string is not generally a useful
value in the xDS APIs, so can be be considered equivalent to an unset
field. This is now documented in principles in #28 as well.
Also replace google.protobuf.BytesValue with bytes.
Piotr Sikora
htuch