proxy/handler: send logs via the rust log crate

[jkarneges]

Currently, when calling C++ code from Rust, any logging performed by the C++ code won't behave properly unless the C++ logger is also initialized. We have similar problems going the other direction. This PR attempts to consolidate all logging from either language to use the Rust logger by:

• Adding FFI functions for initializing the Rust logger and writing messages to it.
• Updating the C++ logging functions to write log messages to the Rust logger if it has been initialized.
• Updating proxy and handler to initialize the Rust logger at startup instead of the C++ logger.

If the Rust logger is not initialized then the C++ logging functions will behave based on the C++ logger configuration. The only programs left doing this are the legacy runner and m2adapter, which are deprecated.