* add TcpProxyClient and TcpProxyServer abstractions to SDK
* add single connection example
* add multi-connection example
* add simple echo server to `tools/`: used for multi-connection example
* update FFI toml files: switched to local imports
* add proxy bindings to `ffi/shared`
* add proxy bindings and example to `ffi/go`
* add note to `ffi/cpp` about lack of Proxy bindings for the moment
mfahampshire