As part of this change, we ensure that the SAN extensions are marked as
critical when the subject is empty so that AWS PCA tolerates the loss of
common names well and continues to function as a Connect CA provider.
Parts of this currently hack around a bug in crypto/x509 and can be
removed after https://go-review.googlesource.com/c/go/+/329129 lands in
a Go release.
Note: the AWS PCA tests do not run automatically, but the following
passed locally for me:
ENABLE_AWS_PCA_TESTS=1 go test ./agent/connect/ca -run TestAWS
