minor #15910 [HttpFoundation] 14114 header option 'X-Forwarded-Prefix' (JohJohan)

This PR was submitted for the 5.2 branch but it was merged into the 5.3 branch instead.

Discussion
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[HttpFoundation] 14114  header option 'X-Forwarded-Prefix'

Fixes #14114.

Should i also document something about using it with traefik? `@jeff1985` you might be able to anwser that as you added the code with symfony/symfony#37734

Could a label `hacktoberfest-accepted` be added to this pull request? I am participating https://hacktoberfest.digitalocean.com/details

Commits
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c670141 14114 [HttpFoundation] header option 'X-Forwarded-Prefix'

Author: javiereguiluz

deployment/proxies.rst

@@ -34,7 +34,7 @@ and what headers your reverse proxy uses to send information:
             # the IP address (or range) of your proxy
             trusted_proxies: '192.0.0.1,10.0.0.0/8'
             # trust *all* "X-Forwarded-*" headers
-            trusted_headers: ['x-forwarded-for', 'x-forwarded-host', 'x-forwarded-proto', 'x-forwarded-port']
+            trusted_headers: ['x-forwarded-for', 'x-forwarded-host', 'x-forwarded-proto', 'x-forwarded-port', 'x-forwarded-prefix']
             # or, if your proxy instead uses the "Forwarded" header
             trusted_headers: ['forwarded']
 
@@ -59,6 +59,7 @@ and what headers your reverse proxy uses to send information:
                 <framework:trusted-header>x-forwarded-host</framework:trusted-header>
                 <framework:trusted-header>x-forwarded-proto</framework:trusted-header>
                 <framework:trusted-header>x-forwarded-port</framework:trusted-header>
+                <framework:trusted-header>x-forwarded-prefix</framework:trusted-header>
 
                 <!-- or, if your proxy instead uses the "Forwarded" header -->
                 <framework:trusted-header>forwarded</framework:trusted-header>
@@ -75,7 +76,7 @@ and what headers your reverse proxy uses to send information:
                 // the IP address (or range) of your proxy
                 ->trustedProxies('192.0.0.1,10.0.0.0/8')
                 // trust *all* "X-Forwarded-*" headers (the ! prefix means to not trust those headers)
-                ->trustedHeaders(['x-forwarded-for', 'x-forwarded-host', 'x-forwarded-proto', 'x-forwarded-port'])
+                ->trustedHeaders(['x-forwarded-for', 'x-forwarded-host', 'x-forwarded-proto', 'x-forwarded-port', 'x-forwarded-prefix'])
                 // or, if your proxy instead uses the "Forwarded" header
                 ->trustedHeaders(['forwarded'])
             ;