[BENG-40] ec2util: future-proof aws identitiy document verify

Summary:
AWS Identity Documents are signed with DSA-with-SHA1 signatures, and DSA is
removed from x509/crypto as of Go 1.16. This follows an approach done
by hashicorp/vault#12340, by forking in
https://github.com/mozilla-services/pkcs7 and applying the DSA fix
mozilla-services/pkcs7#50.

Test Plan: - Tested Bazel build and go build from within the folder

Reviewers: aeiser, dborcherding, sdunn, anguyen, gvitta, bbentson

Reviewed By: dborcherding, sdunn

Subscribers: jcharumilind, dnicolaou, pboyapalli, smahadevan

Differential Revision: https://phabricator.grailbio.com/D66326

fbshipit-source-id: cafa39b

cloud/ec2util/ec2util.go

@@ -22,8 +22,8 @@ import (
 	"time"
 
 	"github.com/aws/aws-sdk-go/service/ec2"
-	"github.com/fullsailor/pkcs7"
 	"v.io/x/lib/vlog"
+	"go.mozilla.org/pkcs7"
 )
 
 type IdentityDocument struct {