Support the selection of X-Forwarded-For value

[shawkins]

Description

See keycloak/keycloak#13261 - some users are behind proxies that don't support overwriting the X-Forwarded-For value, and instead append to it by default. Without introducing yet another proxy to control the rewrite of the X-Forwarded-For, they cannot configure through Keycloak which value to use - Quarkus only selects the left most value, which could even have been set by the end client in these scenarios.

The users would like a way to select the Nth value from the right rather than only getting the left most value.

Implementation ideas

No response

[gsmet]

Maybe we should even support both approaches described in https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-For#selecting_an_ip_address : proxy count and proxy lists.

Let's maybe draft a design on how to configure this before implementing it.

[quarkus-bot]

/cc @pedroigor (bearer-token), @sberyozkin (bearer-token,jwt,security)

[sberyozkin]

We can probably support custom selectors