Feature: Add new PSR-7 implementation and remove dated RingCentral PSR-7 dependency. (#518, #519, #520 and #522 by @clue)

This changeset allows us to maintain our own PSR-7 implementation and reduce dependencies on external projects. It also improves performance slightly and does not otherwise affect our public API. If you want to explicitly install the old RingCentral PSR-7 dependency, you can still install it like this:

composer require ringcentral/psr7

Feature: Add new Uri class for new PSR-7 implementation. (#521 by @clue)

Feature: Validate outgoing HTTP message headers and reject invalid messages. (#523 by @clue)

Feature: Full PHP 8.3 compatibility. (#508 by @clue)

Fix: Fix HTTP client to omit Transfer-Encoding: chunked when streaming empty request body. (#516 by @clue)

Fix: Ensure connection close handler is cleaned up for each request. (#515 by @WyriHaximus)

Update test suite and avoid unhandled promise rejections. (#501 and #502 by @clue)