We now provide a configurable way for dynamic proxy route selection
for the default HttpClient based on regex based targetHostname patterns.
Introduced `ProxyMapping` to describe a regex based mapping
between target hosts and the proxy URL to use.
A `ProxyMapping` can be build from an ordered list of string based
mapping representations, e.g:
```
^.*.(google.com|googleapis.com)$;http://localhost:8080
```
If the targetHost does not match a configured proxy mapping,
no proxy is used.
This can be configured via standalone.xml / jboss-cli, e.g.:
```
echo SETUP: Configure proxy routes for HttpClient SPI
/subsystem=keycloak-server/spi=connectionsHttpClient/provider=default:add(enabled=true)
/subsystem=keycloak-server/spi=connectionsHttpClient/provider=default:write-attribute(name=properties.proxy-mappings,value=["^.*.(google.com|googleapis.com)$;http://www-proxy1:8080","^.*.facebook.com$;http://www-proxy2:8080"])
```
The new `ProxyMappingWareRoutePlanner` uses a configured `ProxyMapping`
to decide which proxy to use for a given request based on the target host
denoted by the HTTP request to execute.
I verified this manually with the BurpProxy Suite.
