[HttpFoundation] Issue 7349: Split getClientIp into two methods for better flexibility

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | not locally (2 ICU tests failing, see https://gist.github.com/gnutix/5345173). waiting on Travis results
| Fixed tickets | #7349
| License       | MIT
| Doc PR        | none

Split the ``Request::getClientIp`` method is two to allow better overriding flexibility. See #7349 for more information.

As the PR adds a new method `getClientIps` to the API, should I write something in a documentation/changelog/other about it ?