feat(auth): refactor auth, add oidc

Thanks to all HedgeDoc team members for the time discussing,
helping with weird Nest issues, providing feedback
and suggestions!

Co-authored-by: Philip Molares <philip.molares@udo.edu>
Signed-off-by: Philip Molares <philip.molares@udo.edu>
Signed-off-by: Erik Michelson <github@erik.michelson.eu>

backend/src/users/username-check.dto.ts

@@ -0,0 +1,21 @@
+/*
+ * SPDX-FileCopyrightText: 2024 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import { IsBoolean, IsLowercase, IsString } from 'class-validator';
+
+import { BaseDto } from '../utils/base.dto.';
+import { Username } from '../utils/username';
+
+export class UsernameCheckDto extends BaseDto {
+  // eslint-disable-next-line @darraghor/nestjs-typed/validated-non-primitive-property-needs-type-decorator
+  @IsString()
+  @IsLowercase()
+  username: Username;
+}
+
+export class UsernameCheckResponseDto extends BaseDto {
+  @IsBoolean()
+  usernameAvailable: boolean;
+}