Add PipesDefaultLogWriter

[danielgafni]

Add a log writer which would send stdout/stderr to Dagster via Pipes messages (log_stdio method).

This LogWriter will be useful in some execution environments which don't have a straightforward way of exposing logs.

Python references:

• abstract PipesLogWriter class
• abstract PipesStdioLogWriter class which is responsible for capturing stdout/stderr
• finally, PipesDefaultLogWriter implementation