Module http::oauth2

OAuth2 client (authorization-code + refresh grants, RFC 6749).

Provider-agnostic: every function here just POSTs 'application/x-www-form-urlencoded' to whatever token endpoint host/port/path you give it, so it works against any real OAuth2 provider's token endpoint (Google, GitHub, a self-hosted authorization server, ...) — nothing here is tied to one provider. This is the token-exchange half only; building the authorization redirect URL (the browser-facing '/authorize?...' step) is one string-format call away and deliberately left to the caller, since it's provider-specific query parameters, not a protocol exchange.