Pull request 2111: 6545-schema-version

Updates #6545.

Squashed commit of the following:

commit b1969128a99ff21c97feb4e7805b4b8133d7122f
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Dec 15 20:04:37 2023 +0300

    home: fix import

commit 872ccea1491a8da76cc24db79247438d0ce4d256
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Dec 15 20:01:15 2023 +0300

    all: output schema version

internal/configmigrate/yaml.go

@@ -0,0 +1,68 @@
+package configmigrate
+
+import (
+	"fmt"
+)
+
+type (
+	// yarr is the convenience alias for YAML array.
+	yarr = []any
+
+	// yobj is the convenience alias for YAML key-value object.
+	yobj = map[string]any
+)
+
+// fieldVal returns the value of type T for key from obj.  Use [any] if the
+// field's type doesn't matter.
+func fieldVal[T any](obj yobj, key string) (v T, ok bool, err error) {
+	val, ok := obj[key]
+	if !ok {
+		return v, false, nil
+	}
+
+	if val == nil {
+		return v, true, nil
+	}
+
+	v, ok = val.(T)
+	if !ok {
+		return v, false, fmt.Errorf("unexpected type of %q: %T", key, val)
+	}
+
+	return v, true, nil
+}
+
+// moveVal copies the value for srcKey from src into dst for dstKey and deletes
+// it from src.
+func moveVal[T any](src, dst yobj, srcKey, dstKey string) (err error) {
+	newVal, ok, err := fieldVal[T](src, srcKey)
+	if !ok {
+		return err
+	}
+
+	dst[dstKey] = newVal
+	delete(src, srcKey)
+
+	return nil
+}
+
+// moveSameVal moves the value for key from src into dst.
+func moveSameVal[T any](src, dst yobj, key string) (err error) {
+	return moveVal[T](src, dst, key, key)
+}
+
+// coalesceError returns the first non-nil error.  It is named after function
+// COALESCE in SQL.  If all errors are nil, it returns nil.
+//
+// TODO(e.burkov): Replace with [errors.Join].
+//
+// TODO(a.garipov): Think of ways to merge with [aghalg.Coalesce].
+func coalesceError(errors ...error) (res error) {
+	for _, err := range errors {
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}