Today settings can declare dependencies on another setting. This
declaration is implemented so that if the declared setting is not set
when the declaring setting is, settings validation fails. Yet, in some
cases we want not only that the setting is set, but that it also has a
specific value. For example, with the monitoring exporter settings, if
xpack.monitoring.exporters.my_exporter.host is set, we not only want
that xpack.monitoring.exporters.my_exporter.type is set, but that it is
also set to local. This commit extends the settings infrastructure so
that this declaration is possible. The use of this in the monitoring
exporter settings will be implemented in a follow-up.
bfb2dc1353