[HttpFoundation] Added Request::getEncodings() method

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

We needed access to the Accept-Encoding information in Drupal, and I was surprised to see that there wasn't a method on the request object to access this.

This PR adds that method.