Remove go-playground/validator dependency from pkg/pki (#1817)

See sigstore/sigstore-go#23.

It might seem silly, but this reduces the size of sigstore-go by over 1 MB.
We're already using asaskevich/govalidator elsewhere in Rekor, so no new
dependencies added.

If we wanted to remove go-playground/validator entirely from Rekor (which
would shrink rekor-cli by over 1 MB as well), we'd need additional work
to rekor-cli/app/ files pflags.go and validate.go.

Signed-off-by: Zach Steindler <steiza@github.com>

pkg/pki/pgp/pgp.go

@@ -28,7 +28,7 @@ import (
 	"io"
 	"net/http"
 
-	validator "github.com/go-playground/validator/v10"
+	"github.com/asaskevich/govalidator"
 
 	//TODO: https://github.com/sigstore/rekor/issues/286
 	"golang.org/x/crypto/openpgp"        //nolint:staticcheck
@@ -295,9 +295,7 @@ func (k PublicKey) EmailAddresses() []string {
 	// Extract from cert
 	for _, entity := range k.key {
 		for _, identity := range entity.Identities {
-			validate := validator.New()
-			errs := validate.Var(identity.UserId.Email, "required,email")
-			if errs == nil {
+			if govalidator.IsEmail(identity.UserId.Email) {
 				names = append(names, identity.UserId.Email)
 			}
 		}

pkg/pki/x509/x509.go

@@ -28,7 +28,7 @@ import (
 	"io"
 	"strings"
 
-	validator "github.com/go-playground/validator/v10"
+	"github.com/asaskevich/govalidator"
 	"github.com/sigstore/rekor/pkg/pki/identity"
 	"github.com/sigstore/sigstore/pkg/cryptoutils"
 	sigsig "github.com/sigstore/sigstore/pkg/signature"
@@ -184,10 +184,8 @@ func (k PublicKey) EmailAddresses() []string {
 		cert = k.certs[0]
 	}
 	if cert != nil {
-		validate := validator.New()
 		for _, name := range cert.EmailAddresses {
-			errs := validate.Var(name, "required,email")
-			if errs == nil {
+			if govalidator.IsEmail(name) {
 				names = append(names, strings.ToLower(name))
 			}
 		}