Add Firebase Data Connect support

[lisajian]

Description

Adds basic FDC function trigger for onMutationExecuted

uv run pytest passes locally

Internal tracking bug: b/436623435

Code sample

from firebase_functions import dataconnect_fn

@dataconnect_fn.on_mutation_executed(
  service="service-id",
  connector="connector-id",
  operation="mutation-name"
):
def mutation_executed_handler(event: dataconnect_fn.Event):
  pass

See go/fdc-eventarc-functions-sdk for more information