feat: oxidize open_stream

[bepri]

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This is based on (and thus requires) #411. The diff will be pretty noisy until it is merged.

I recommend not reviewing the code directly until I've had an opportunity to clean up the commit history once #411 lands, but this can be tested using the scripts below in the meantime.

This PR adds the open_stream method back to the emitter. This required refactoring the Printer class to not solely exist as a field on an Emitter instance, as the stream handler object needed access too. Now, the Printer is a global, static variable that gets initialized upon the first print. For similar reasons, the logging mechanism has been moved into the printer and must be explicitly initialized via the new Emitter.init_logger().

Here is some example code to test the changes:

# testing.py
import subprocess

from craft_cli import EmitterMode
from craft_cli._rs.emitter import Emitter

e = Emitter("blah.log", EmitterMode.TRACE, "example.com", "hello from rust!")
e.init_logger()

with e.open_stream() as stream:
    subprocess.run(["bash", "testing.sh"], stdout=stream, stderr=stream)

# testing.sh
for i in {1..6}; do
  echo $i
  sleep 1
done

Known bugs: It seems that trace-level events (and perhaps others?) duplicate the timestamp in logs (tracked in CRAFT 4999)

CRAFT-4964.