Title: OAuth2: remove internal cookies before forwarding request to backend

Description:
The cookies other than AccessToken and IDToken should be deleted before forwarding the request to the Backend applications since they're supposed to be only used by the OAuth2 filter itself.

Discussion in Envoy Gateway:

• Option to overwrite every OAuth2Filter Cookie via OIDCCookieNames gateway#5152 (comment)
• api(oidc): Allow override of all OIDC CookieNames gateway#5655 (comment)