[DomCrawler] adds schema relative URL support to link

Adds support for `//relative/schema` URLs to `DomCrawler` links.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7169
| License       | MIT