PT46890219: add internal caller-id rewrite in UI

- allow config plugins to contribute proxy hooks in proxy configuration file

PT46890219: add internal caller-id rewrite in UI

- link sipXcustomCallerId as git subrepo

PT46890219: Fix sipXcustomcallerId RPM build

PT46890219: sixcustomcallerid - fix sipXproxy-config file generation

PT50262531 - Proxy with customCallerId plugin loaded dies when calls are placed

UtlString constructor from std::string was using uninitialized internal data.