[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?