Allow setting domain config override

With this you can inject the domainConfigOverrider (domain_config.overrider) service and then do:

// Load the domain you want.
$domain = $this->myFunctionToLoadADomain($params);
$this->domainConfigOverrider->setDomain($domain);

I have tested this by creating an entity from my primary domain with settings on the entity that relate to two different domains. I have an event subscriber on entity insert that determines the domain and triggers a MailChimp campaign. The MailChimp module successfully loads the config for the relevant domain.

Merged, thanks!

You might have to call initiateContext() before using the setDomain() method.