Build the application/x-www-form-urlencoded body for an upstream
refresh-token exchange (OAuth 2.1 §4.3 / RFC 6749 §6). Carries
grant_type=refresh_token, the client-relayed refresh_token, the proxy's
fixed upstream client_id, and (RFC 8707) resource. For
client_secret_post the upstream secret is appended to the body; for
client_secret_basic it is sent via proxyTokenAuthHeader instead.
Build the application/x-www-form-urlencoded body for an upstream refresh-token exchange (OAuth 2.1 §4.3 / RFC 6749 §6). Carries grant_type=refresh_token, the client-relayed refresh_token, the proxy's fixed upstream client_id, and (RFC 8707) resource. For client_secret_post the upstream secret is appended to the body; for client_secret_basic it is sent via proxyTokenAuthHeader instead.