feat(core): Replace client-oauth2 with an in-repo package (#6266)

Co-authored-by: Marcus <marcus@n8n.io>

packages/@n8n/client-oauth2/test/config.ts

@@ -0,0 +1,15 @@
+export const baseUrl = 'https://mock.auth.service';
+export const accessTokenUri = baseUrl + '/login/oauth/access_token';
+export const authorizationUri = baseUrl + '/login/oauth/authorize';
+export const redirectUri = 'http://example.com/auth/callback';
+
+export const accessToken = '4430eb1615fb6127cbf828a8e403';
+export const refreshToken = 'def456token';
+export const refreshedAccessToken = 'f456okeendt';
+export const refreshedRefreshToken = 'f4f6577c0f3af456okeendt';
+
+export const clientId = 'abc';
+export const clientSecret = '123';
+
+export const code = 'fbe55d970377e0686746';
+export const state = '7076840850058943';