Previously the adapter configuration did not support specifying a proxy
for auth-server requests issued via the Apache HTTP Client by Keycloak.
This made it very difficult to connect an Application with Keycloak
that was required to use a proxy.
Introduced new `proxy-url` attribute to the adapter configuration
which makes it possible to configure a proxy to be used for auth-server
requests. Proxy-Host, Proxy-Port and Proxy-Scheme are taken from the
configured proxy URL.
Note that proxies that require authentication are currently not supported.
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