feat: rename "reverse proxy authentication" to "external authentication" (#4418)

* Rename external auth options

ReverseProxyWhitelist was regularly confusing users that enabled it for
non-authenticating reverse proxy setups.

The new option name makes it clear that it's related to authentication, not
just reverse proxies.

* small refactor

Signed-off-by: Deluan <deluan@navidrome.org>

* add test

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>
Co-authored-by: Deluan Quintão <deluan@navidrome.org>

log/log.go

@@ -29,8 +29,8 @@ var redacted = &Hook{
 		"(Secret:\")[\\w]*",
 		"(Spotify.*ID:\")[\\w]*",
 		"(PasswordEncryptionKey:[\\s]*\")[^\"]*",
-		"(ReverseProxyUserHeader:[\\s]*\")[^\"]*",
-		"(ReverseProxyWhitelist:[\\s]*\")[^\"]*",
+		"(UserHeader:[\\s]*\")[^\"]*",
+		"(TrustedSources:[\\s]*\")[^\"]*",
 		"(MetricsPath:[\\s]*\")[^\"]*",
 		"(DevAutoCreateAdminPassword:[\\s]*\")[^\"]*",
 		"(DevAutoLoginUsername:[\\s]*\")[^\"]*",