Optionally display a flag saying the file came from a proxy cache

[twarit-waikar]

When using p4-fusion with a Perforce proxy server, it is sometimes beneficial for performance debugging purposes to know the proxy cache hit/miss rate while downloading files through p4 print

This is usually done by adding a -Zproxyverbose tag in the p4 CLI:

p4 -Zproxyverbose sync

We can specify the same tag when we create the P4API contexts. https://github.com/salesforce/p4-fusion/blob/6746b6065719517b10739d5eda87a2c906822fcd/p4-fusion/p4_api.cc#L50

And then we should be able to log that information through p4-fusion, either simply to stdout or add it in the commit message