Support runtime hostname injection

If a request to the CMS application is received and there is no corresponding host in the HST hosts configuration for /hst:platform, auto-generate in memory a host group configuration for the platform and all registered HST site webapps. This host config should by default provide equivalent behavior to the current dev-localhost host group except for the ingress hostname. There should also be a configuration option to allow a different host group to be used as the template, e.g. via hst.properties and the various fallback mechanisms. It should also be possible to explicitly disable this behavior if it is undesired.

A request to a site application that does not match a host should NOT auto-generate a host group, as we consider this too risky in case of misconfiguration of the HTTP proxy layer. In that case, the current error-reporting behavior should be retained.