Redaction in Rust

Use the standard fmt::Debug ("{:?}" in format strings) on Protobuf messages for human-readable strings for logging, error messages, exceptions, and similar use cases. The output of this debug info is not intended to be machine-readable (unlike TextFormat and JSON which are not be used for debug output).

Using fmt::Debug enables redaction of some sensitive fields.

Note that under upb kernel this redaction is not yet implemented, but is expected to be added.