Allow a forwarding proxy to intergate an external script/process action before a forwarding action

When one deploys a Stroom proxy in a hostile environment, it would be beneficial for an anti-virus or similar capability be applied against the received data before further processing.

In the case of Store No-DB or Store Proxies, this can be achieved simply. In the case of a forwarding proxy, where one can aggregate and forward, there would be some benefit to be able to configure a call to say, an external process, before the forwarding action occurs. Should the process return a 0 exit status, the proxy proceeds with the forwarding process, if non-zero, then it does not proceed. Naturally, this configuration option may only be applied when the forwarding is not immediate and an error should occur if this configuration is attempted.

No files, tests, or entry points are named. Start by locating the forwarding proxy configuration and the non-immediate forwarding path, then determine how an external process could gate forwarding. Done should include proceeding on exit status 0, stopping on non-zero status, and rejecting the configuration for immediate forwarding.

Written by the indexing model from the issue text.