Implement PSR-7 and PSR-17

[jakejackson1]

As part of standardising this library, it would be good to create a new major release that implemented the PSR-7 UploadedFileInterface and the UploadedFileFactoryInterface PSR-17 factory.

If we can, rather than extend the interface ourselves I'd recommend leveraging an existing well-tested library. Possible contenders include:

https://github.com/Nyholm/psr7
https://github.com/nimbly/Capsule
https://github.com/slimphp/Slim-Psr7
https://github.com/guzzle/psr7/
https://github.com/httpsoft/http-message (PHP7.4+)