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/testdata/cfg.json

@@ -1,6 +1,7 @@
 {
   "musicFolder": "/json/music",
   "uiWelcomeMessage": "Welcome json",
+  "reverseProxyUserHeader": "X-Auth-User",
   "Tags": {
     "artist": {
       "split": ";"