proxy client: allow to configure user agent for proxy

c/opendht.cpp

c/opendht_c.h

include/opendht/dht_proxy_client.h

include/opendht/dhtrunner.h

python/opendht.pyx

python/opendht_cpp.pxd

rust/src/dhtrunner.rs

rust/src/ffi.rs

src/dht_proxy_client.cpp

src/dhtrunner.cpp