[CT-1928] Support passing event callbacks to EventManager via dbtRunner

[MichelleArk]

Accept event callbacks as part of dbtRunner's constructor and use them to setup the EventManager for programmatic invocations of dbt.

[stu-k]

My have to add a method to add a callback to EventManager, and use the ctx to carry the callbacks from dbtRunner's constructor to preflight where we setup the logger.

[chamini2]

Hey, the current implementation in #7214 seems to wait for the callbacks to be processed before continuing with the invocation. Let me share an example:

from dbt.cli.main import dbtRunner, list
from dbt.events.base_types import EventMsg

import time

def print_callback(event: EventMsg):
    print("Sleeping")
    time.sleep(2)
    print("Got an event:", event.info.name, event)

dbt = dbtRunner(callbacks=[print_callback])
dbt.invoke(['list'])

This takes 30 seconds to run because it waits for the time.sleep(2) to finish befor continuing.

Is this an implementation detail that may change in the future or as intended?

[DanielDuranSW]

Hello!! Do you know about the future support for registering callbacks on dbt's EventManager? "The current behavior blocks subsequent steps, but this may not be guaranteed in future versions". "Given that the Python interface for events is less mature than the structured logging interface", would it be better to rely on parsing JSON-formatted logs for handling real-time events, or are callbacks still the recommended approach for this use case? @MichelleArk