fix: Revert "Replace client-oauth2 with an in-repo package" (no-changelog) (#6265)

Revert "feat(core): Replace client-oauth2 with an in-repo package (#6056)"

This reverts commit 77ac953eaf.

packages/@n8n/client-oauth2/src/CredentialsFlow.ts

@@ -1,52 +0,0 @@
-import type { ClientOAuth2, ClientOAuth2Options } from './ClientOAuth2';
-import type { ClientOAuth2Token, ClientOAuth2TokenData } from './ClientOAuth2Token';
-import { DEFAULT_HEADERS } from './constants';
-import { auth, expects, getRequestOptions, sanitizeScope } from './utils';
-
-interface CredentialsFlowBody {
-	grant_type: 'client_credentials';
-	scope?: string;
-}
-
-/**
- * Support client credentials OAuth 2.0 grant.
- *
- * Reference: http://tools.ietf.org/html/rfc6749#section-4.4
- */
-export class CredentialsFlow {
-	constructor(private client: ClientOAuth2) {}
-
-	/**
-	 * Request an access token using the client credentials.
-	 */
-	async getToken(opts?: Partial<ClientOAuth2Options>): Promise<ClientOAuth2Token> {
-		const options = { ...this.client.options, ...opts };
-
-		expects(options, 'clientId', 'clientSecret', 'accessTokenUri');
-
-		const body: CredentialsFlowBody = {
-			grant_type: 'client_credentials',
-		};
-
-		if (options.scopes !== undefined) {
-			body.scope = sanitizeScope(options.scopes);
-		}
-
-		const requestOptions = getRequestOptions(
-			{
-				url: options.accessTokenUri,
-				method: 'POST',
-				headers: {
-					...DEFAULT_HEADERS,
-					// eslint-disable-next-line @typescript-eslint/naming-convention
-					Authorization: auth(options.clientId, options.clientSecret),
-				},
-				body,
-			},
-			options,
-		);
-
-		const responseData = await this.client.request<ClientOAuth2TokenData>(requestOptions);
-		return this.client.createToken(responseData);
-	}
-}