fix(OAuth2API): enable token exchange without token (discordjs#10312)

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Author: Gravitonic

packages/core/src/api/oauth2.ts

@@ -47,6 +47,7 @@ export class OAuth2API {
 			headers: {
 				'Content-Type': 'application/x-www-form-urlencoded',
 			},
+			auth: false,
 			signal,
 		}) as Promise<RESTPostOAuth2AccessTokenResult>;
 	}
@@ -68,6 +69,7 @@ export class OAuth2API {
 			headers: {
 				'Content-Type': 'application/x-www-form-urlencoded',
 			},
+			auth: false,
 			signal,
 		}) as Promise<RESTPostOAuth2RefreshTokenResult>;
 	}
@@ -91,6 +93,7 @@ export class OAuth2API {
 			headers: {
 				'Content-Type': 'application/x-www-form-urlencoded',
 			},
+			auth: false,
 			signal,
 		}) as Promise<RESTPostOAuth2ClientCredentialsResult>;
 	}