Make log-collector rsyslog configuration extensible

We'd like to be able to add custom diversions of certain log flows to specific Elasticsearch indexes (for example for the long-term user account audit logs). To do so we need to add logic in the main rsyslog-collector configuration.

One possibility would be to explore splitting the current configuration into pieces in a conf.d directory, which may then allow us to add custom logic just by dopping a file. This might require altering the control flow of the current configuration to make it more linear and composible.