Expression language extensibility

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10512
| License       | MIT
| Doc PR        | not yet

The way we can add functions to an ExpressionLanguage instance is by using inheritance. #10512 tries to make the expression language in the routing flexible but using inheritance won't work when several bundles want to add functions.

So, this PR takes another approach to solve the problem globally.

Todo:

 * [ ] add some more tests
 * [ ] add some docs


``ExpressionFunction[]``