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>

conf/configuration_test.go

@@ -41,6 +41,9 @@ var _ = Describe("Configuration", func() {
 			Expect(conf.Server.Tags["custom"].Aliases).To(Equal([]string{format, "test"}))
 			Expect(conf.Server.Tags["artist"].Split).To(Equal([]string{";"}))
 
+			// Check deprecated option mapping
+			Expect(conf.Server.ExtAuth.UserHeader).To(Equal("X-Auth-User"))
+
 			// The config file used should be the one we created
 			Expect(conf.Server.ConfigFile).To(Equal(filename))
 		},