Looks kind of weird to have the href as an attribute an the route as an element.

What about

```xml
<relation name="bar">
    <href>/foo</href>
</relation>

<relation name="bar">
    <href route="acme_user_get" absolute="true">
        <parameter name="id">expr(object.getId())</parameter>
    </href>
    <embed>
        <content>expr(object.getFoo())</content>
        <exclusion groups="group1, group2" since-version="1" until-version="2.2" max-depth="42" exclude-if="foo"/>
    </embed>
    <exclusion groups="group1, group2" since-version="1" until-version="2.2" max-depth="42" exclude-if="foo"/>
</relation>
```

The thing is that the route parameter can be a nested array in annotations and yaml, same for the embed content. So we should find a format that can represent nested arrays.

And we should not introduce `<expr>` tags because we can also do `<relation name="expr(object.getRelationName())">` and it would be inconsistent.