Add Akeyless vaultless platform support for sigstore

Akeyless is a SaaS vaultless key management platform.

For more info see
https://www.akeyless.io/

pkg/signature/kms/akeyless/akeyless_test.go

@@ -0,0 +1,81 @@
+//
+// Copyright 2021 The Sigstore Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package akeyless
+
+import "testing"
+
+func TestParseReference(t *testing.T) {
+	tests := []struct {
+		in          string
+		wantKey     string
+		wantVersion int32
+		wantErr     bool
+	}{
+		{
+			in:          "akeyless://cosign",
+			wantKey:     "cosign",
+			wantVersion: 0,
+			wantErr:     false,
+		},
+		{
+			in:          "akeyless://cosign/2",
+			wantKey:     "cosign",
+			wantVersion: 2,
+			wantErr:     false,
+		},
+		{
+			in:          "akeyless://cosign/nested",
+			wantKey:     "cosign/nested",
+			wantVersion: 0,
+			wantErr:     false,
+		},
+		{
+			in:          "akeyless://cosign/nested/2",
+			wantKey:     "cosign/nested",
+			wantVersion: 2,
+			wantErr:     false,
+		},
+		{
+			in:      "foo://bar",
+			wantErr: true,
+		},
+		{
+			in:      "akeyless://",
+			wantErr: true,
+		},
+		{
+			in:      "",
+			wantErr: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.in, func(t *testing.T) {
+			gotKey, gotVersion, err := parseReference(tt.in)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("parseReference() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+
+			if gotKey != tt.wantKey {
+				t.Errorf("parseReference() gotKey = %v, want %v", gotKey, tt.wantKey)
+			}
+
+			if gotVersion != tt.wantVersion {
+				t.Errorf("parseReference() gotVersion = %v, want %v", gotVersion, tt.wantVersion)
+			}
+		})
+	}
+}