Enhancement: Support PSR-15 middleware adaption

[sminnee]

Now that SS4 has a robust middleware system, create 1 or more middlwareadapters that let you plug in PSR-7 based middleware.

Since there isn't yet a single convention for these middlewares, we might want to have a few, e.g. SlimMiddlewareAdapter

[robbieaverill]

@sminnee Are you referencing this proposal for the spec?

[sminnee]

I hadn't when I wrote this but that's probably the best bet. The composer package for PSR-15 already exists so you could just tie to that.

[sminnee]

It could also be that you do a few different micropackages, so psr15-middleware depended on psr7-adapters

[robbieaverill]

True. Or maybe this could become psr-adapters and do all the things

[robbieaverill]

Updated (accepted) PSR spec: https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-15-request-handlers.md