vmalert: support configuration file for notifiers
* vmalert notifiers now can be configured via file
see https://docs.victoriametrics.com/vmalert.html#notifier-configuration-file
* add support of Consul service discovery for notifiers config
see https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1947
* add UI section for currently loaded/discovered notifiers
* deprecate `-rule.configCheckInterval` in favour of `-configCheckInterval`
* add ability to suppress logs for duplicated targets for notifiers discovery
* change behaviour of `vmalert_alerts_send_errors_total` - it now accounts
for failed alerts, not HTTP calls.
