[PSR-7] Describe an Incoming Request

This pull request defines an additional interface for PSR-7, the Incoming Request. For server-side applications, a number of request sources are commonly required in order to handle the incoming request, including deserialized query string arguments, file upload metadata, cookies, deserialized message body parameters (if any), and path parameters as matched by routing.

The first three (query params, incoming files, and cookies) are commonly available via `$_GET`,  `$_FILES`, and `$_COOKIE`, although situations exist when those are not present and/or usable (e.g., during testing; when dispatching sub-requests); they SHOULD remain immutable for the duration of a request, however, and thus no setters are required.

In the case of body parameters, `$_POST` _can_ be used, but is often not applicable (e.g., in cases where a non-POST HTTP method with a message body is present; and in cases where an incoming HTTP message body is present but not in a form-encoded content type); as such, a setter is needed so that code can discover the parameters from the incoming request body and inject them.

In the case of path parameters, these will be always specifically dependent on the consuming code, and thus will need to be injected by consumers.

The new interface, `Psr\Http\Message\IncomingRequestInterface`, extends the `Psr\Http\Message\RequestInterface`, and describes accessors and mutators for each of these as required, with accessors being the common API for consumers, and mutators being present for those properties that can only be discovered by introspecting the request..

The following methods are defined:

- `getQueryParams()`
- `getFileParams()`
- `getCookieParams()`
- `setCookieParams()` (Available for cases where libraries perform filtering, such as decryption, on incoming cookies.)
- `getBodyParams()`
- `setBodyParams($params)`
- `getPathParams()`
- `setPathParams($params)`