Epic #83894: PSR-15 Initiative
Introduce PSR-15 HTTP Middleware support
The PSR-15 middleware specification was released on 22nd of january 2018.
TYPO3 should support PSR-15 middlewares out-of-the box.
Supporting PSR-15 middlewares improves interoperability with independent libraries and would superseed ugly hooks like
$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['tslib/index_ts.php']['preprocessRequest'] (ugly as they have not been adapted when PSR-7 Requests/Responses were introduced).