Implement PSR-17/18 adapter for direct connection to Guzzle 7

[BusterNeece]

Hello! The recent release of version 7 of the Guzzle HTTP adapter has introduced direct support from the library for the PSR-18 (HTTP Client) standard. The folks at HTTPlug have said that they don't have immediate plans for adding support for Guzzle 7, since in the long term, the goal is to have PSR-18-compatible libraries just consumed directly, without need for a bridge layer.

Along with this, it would be mighty handy if we could implement an adapter that supports a PSR-17 (Request factory) and a PSR-18 HTTP client, allowing the removal of a fair number of intermediate bridge libraries. Ideally, just fXmlRpc, the PSR-17 request factory, and the PSR-18-implementing library itself would be required (the former and/or the latter requiring the PSR interfaces themselves).

If there's no opposition, I plan to begin working on a pull request for this shortly.