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>

server/subsonic/middlewares.go

@@ -56,7 +56,7 @@ func fromInternalOrProxyAuth(r *http.Request) (string, bool) {
 		return username, true
 	}
 
-	return server.UsernameFromReverseProxyHeader(r), false
+	return server.UsernameFromExtAuthHeader(r), false
 }
 
 func checkRequiredParameters(next http.Handler) http.Handler {

server/subsonic/middlewares_test.go

@@ -95,8 +95,8 @@ var _ = Describe("Middlewares", func() {
 		})
 
 		It("passes when all required params are available (reverse-proxy case)", func() {
-			conf.Server.ReverseProxyWhitelist = "127.0.0.234/32"
-			conf.Server.ReverseProxyUserHeader = "Remote-User"
+			conf.Server.ExtAuth.TrustedSources = "127.0.0.234/32"
+			conf.Server.ExtAuth.UserHeader = "Remote-User"
 
 			r := newGetRequest("v=1.15", "c=test")
 			r.Header.Add("Remote-User", "user")
@@ -254,8 +254,8 @@ var _ = Describe("Middlewares", func() {
 		When("using reverse proxy authentication", func() {
 			BeforeEach(func() {
 				DeferCleanup(configtest.SetupConfig())
-				conf.Server.ReverseProxyWhitelist = "192.168.1.1/24"
-				conf.Server.ReverseProxyUserHeader = "Remote-User"
+				conf.Server.ExtAuth.TrustedSources = "192.168.1.1/24"
+				conf.Server.ExtAuth.UserHeader = "Remote-User"
 			})
 
 			It("passes authentication with correct IP and header", func() {