[2.2] [Routing] add support for path-relative URL generation

Tests pass: yes
Feature addition: yes
BC break: <del>tiny (see below)</del> NO
deprecations: NO

At the moment the Routing component only supports absolute and domain-relative URLs, e.g.
`http://example.org/user-slug/article-slug/comments` and
`/user-slug/article-slug/comments`.

But there are two link types missing: schema-relative URLs and path-relative URLs.
schema-relative: e.g. `//example.org/user-slug/article-slug/comments`
path-relative: e.g. `comments`.

Both of them would now be possible with this PR. I think it closes a huge gap in the Routing component.
Use cases are pretty common. Schema-relative URLs are for example used when you want to include assets (scripts, images etc) in a secured website with HTTPS. Path-relative URLs are the only option when you want to generate static files (e.g. documentation) that can be downloaded as an HTML archive. Such use-cases are currently not possible with symfony.

The calculation of the relative path based on the request path and target path is hightly unit tested. So it is really equivalent. I found several implemenations on the internet but none of them worked in all cases. Mine is pretty short and works.

I also added an optional parameter to the twig `path` function, so this feature can also be used in twig templates.

Ref: This implements path-relative URLs as suggested in #3908.

<del>[BC BREAK] The signature of UrlGeneratorInterface::generate changed to support scheme-relative and path-relative URLs. The core UrlGenerator is BC and does not break anything, but users who implemented their own UrlGenerator need to be aware of this change. See UrlGenerator::convertReferenceType.</del>