cleanup: move KMS functionality into its own package

A new "internal/kms" package is introduced, it holds the API that can be
consumed by the RBD components.

The KMS providers are currently in the same package as the API. With
later follow-up changes the providers will be placed in their own
sub-package.

Because of the name of the package "kms", the types, functions and
structs inside the package should not be prefixed with KMS anymore:

    internal/kms/kms.go:213:6: type name will be used as kms.KMSInitializerArgs by other packages, and that stutters; consider calling this InitializerArgs (golint)

Updates: #852
Signed-off-by: Niels de Vos <ndevos@redhat.com>

internal/kms/kms_test.go

@@ -0,0 +1,55 @@
+/*
+Copyright 2021 The Ceph-CSI 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 kms
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func noinitKMS(args ProviderInitArgs) (EncryptionKMS, error) {
+	return nil, nil
+}
+
+func TestRegisterProvider(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		provider Provider
+		panics   bool
+	}{{
+		Provider{
+			UniqueID: "incomplete-provider",
+		},
+		true,
+	}, {
+		Provider{
+			UniqueID:    "initializer-only",
+			Initializer: noinitKMS,
+		},
+		false,
+	}}
+
+	for _, test := range tests {
+		provider := test.provider
+		if test.panics {
+			assert.Panics(t, func() { RegisterProvider(provider) })
+		} else {
+			assert.True(t, RegisterProvider(provider))
+		}
+	}
+}