feat(core): Add PKCE for OAuth2 (#6324)

* Remove authorization header when empty

* Import pkce

* Add OAuth2 with new grant type to Twitter

* Add pkce logic auto assign authorization code if pkce not defined

* Add pkce to ui and interfaces

* Fix scopes for Oauth2 twitter

* Deubg + pass it through header

* Add debug console, add airtable cred

* Remove all console.logs, make PKCE in th body only when it exists

* Remove invalid character ~

* Remove more console.logs

* remove body inside query

* Remove useless grantype check

* Hide oauth2 twitter waiting for overhaul

* Remove redundant header removal

* Remove more console.logs

* Add comment for code verifier

* Remove uneeded scopes

* Restore client id in callback

* Revert "Add OAuth2 with new grant type to Twitter"

This reverts commit 1c3b331aa1974159d1ffe1a4fbf2050722f0f24c.

* Remove oauth2 from twitter

* Remove properties linked to oauth2

* Fix lodash imports

* remove redundant check

* remove redundant codeVerifier

* patch pkce-challenge to avoid generating `code_verifier` with `~`

* store `codeVerifier` on the DB like `csrfSecret`

* remove unrelated changes

---------

Co-authored-by: Marcus <marcus@n8n.io>
Co-authored-by: कारतोफ्फेलस्क्रिप्ट™ <aditya@netroy.in>

packages/core/src/NodeExecuteFunctions.ts

@@ -1104,14 +1104,12 @@ export async function requestOAuth2(
 	});
 
 	let oauthTokenData = credentials.oauthTokenData as ClientOAuth2TokenData;
-
 	// if it's the first time using the credentials, get the access token and save it into the DB.
 	if (
 		credentials.grantType === OAuth2GrantType.clientCredentials &&
 		(oauthTokenData === undefined || Object.keys(oauthTokenData).length === 0)
 	) {
 		const { data } = await getClientCredentialsToken(oAuthClient, credentials);
-
 		// Find the credentials
 		if (!node.credentials?.[credentialsType]) {
 			throw new Error(
@@ -1150,7 +1148,6 @@ export async function requestOAuth2(
 	if (oAuth2Options?.keepBearer === false && typeof newRequestHeaders.Authorization === 'string') {
 		newRequestHeaders.Authorization = newRequestHeaders.Authorization.split(' ')[1];
 	}
-
 	if (oAuth2Options?.keyToIncludeInAccessTokenHeader) {
 		Object.assign(newRequestHeaders, {
 			[oAuth2Options.keyToIncludeInAccessTokenHeader]: token.accessToken,
@@ -1166,7 +1163,9 @@ export async function requestOAuth2(
 				if (oAuth2Options?.includeCredentialsOnRefreshOnBody) {
 					const body: IDataObject = {
 						client_id: credentials.clientId as string,
-						client_secret: credentials.clientSecret as string,
+						...(credentials.grantType === 'authorizationCode' && {
+							client_secret: credentials.clientSecret as string,
+						}),
 					};
 					tokenRefreshOptions.body = body;
 					tokenRefreshOptions.headers = {