[Mercure] Allow placeholder configuration for SCHEMA_HOST of client URL

[fashxp]

see also direct edit here https://docs.pimcore.com/platform/Direct_Edit/Installation/Mercure_Setup#configure-mercure-urls-optional

pimcore_direct_edit:
    mercure_settings:

        # URL of mercure accessible for client. If not set, default will be set to 'http(s)://<PIMCORE_HOST>/hub/.well-known/mercure'. Is is possible to use '<PIMCORE_SCHEMA_HOST>' as placeholder for current schema and host.
        client_side_url:      null

        # URL of mercure accessible for server. If not set, default will be set to 'http(s)://<PIMCORE_HOST>/hub/.well-known/mercure'.
        server_side_url:      null