The change adds no new functionality and aims to move flags definitions
to subpackages that are using them. This should improve readability
of the main function.
app/vmalert: add support for TLS configuration
Add support for TLS optional configuration in a similar fashion to what
is currently supported in other vmutils such as vmagent. TLS
configuration options are distinct for datasource, remoteRead,
remoteWrite as well as notifier.
app/vmalert: Support custom URL for alerts source
Add flag `external.alert.source` for configuring custom URL
for alert's source. This may be handy to re-point default source
URL to other systems like Grafana.
Updates #517
