bindings: plumb tracing into language-native logging

Want some pre-baked functionality to define language-specific tracing subscribers so we can get tailscale-rs tracing output without stepping on the language-specific stuff. I have this marked as an enhancement, but it verges on bugfix, imo, as you're basically blind without a capability like this.

In python you might have something like:

import tailscale
import logging

TAILSCALE_LOGGER = logging.getLogger('tailscale')

def my_trace_logger(level, message, fields):
    TAILSCALE_LOGGER.log(level, message, **fields) # illustrative -- the mapping is going to be more involved

tailscale.bind_tracer(my_trace_logger)

With a similar idea per language: give each the ability to bind a custom function to do whatever they want with.

pre-baked convenience converters

It'd be nice if we provided batteries-included converters for the most popular logging frameworks in each language, e.g.:

import tailscale

tailscale.bind_tracer(tailscale.LOGGING_TRACER)

The issue names no files, tests, or binding entry points. Start by locating the repository's language bindings and existing tracing setup, then determine how a per-language callback and convenience converters would fit each interface. Done means custom tracing handlers can be bound for the supported languages and the proposed logging behavior is defined and covered.