Implement repeatable multi-sources

[jannden]

Suggestion by @sirewix from here:

If our code deals with multiple sources in the same run, it doesn't make sense why there could be at most one source of the same kind, but few of different kinds.

That is, instead of:

pub struct Config {
	pub sources: SourcesConfig,
	// ...
}
pub struct SourcesConfig {
	pub ldap: Option<LdapSourceConfig>,
	pub ukt: Option<UktSourceConfig>,
	pub csv: Option<CsvSourceConfig>,
}

To have something like:

pub struct Config {
	pub sources: Vec<AnySourceConfig>,
	// ...
}

enum AnySourceConfig {
  Ldap(LdapSourceCofnig),
  Ukt(UktSourceConfig),
  Csv(CsvSourceConfig),
}