[HttpFoundation][FrameworkBundle] Add CIDR notation support in trusted proxy list

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | TODO

Added CIDR notation support as requested in #7262.

Also, as long as we need to check against a list of IPs at least in 3 places:
- there is already a loop in HttpKernel's [FragmentListener](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/EventListener/FragmentListener.php#L83),
- the one (added) by this PR,
- and another one (added) by #7251,

does it make sense to modify IpUtils' methods to also accept array as second argument?