Add support of Mercure running on a subdomain

[vrobert78]

On Pimcore PaaS, we'd like to switch from running Mercure in a side app to running Mercure as a Service.
But running Mercure as a Service implies that we run it on a subdomain.
Those modifications are required for the authentication layer.

There are also changes required in studio-backend-bundle, see pimcore/studio-backend-bundle#1394.

[vrobert78]

Regarding the quality gate, I haven't changed the code as it's not mine.
But it should be easy by just changing:

this.topics.forEach(topic => {
  url.searchParams.append('topic', topic)
})

by

for (const topic of this.topics) {
  url.searchParams.append('topic', topic)
}

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
1 Accepted issue

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[markus-moser]

@vrobert78 Thanks a lot for your PR, added the same to a second relevant location (484c83b). I accepted the quality gate case for now as it is really a minor topic. ;)

Will merge it, thanks again!