Bulter.py: Adds preprocess command for local preprocess

[IvanBM18]

Adds preprocess butler script

This command allows developers to generate the serialized and compressed uworker_input payload, upload it to real GCS, and get the signed download URL, exactly as it happens remotely. We can then use said url to trigger a task in any backend that we want:

• In swarming trough a prpc request
• In batch trough manually posting the task to the utask_main queue
  This accelerates local debugging of the tworker preprocessing phase without relying on remote execution queues.

Note: To use this command you need the Secret Manager Secret Accessor for Dev or setup a service account in your local that has those permissions.

Changes

• Added the preprocess subcommand.
  • It interacts with the actual Datastore and GCS based on the provided configuration.
  • It fetches and populates the uworker_env with:
    • Job-specific environment variables from the Datastore.
    • Fuzzer-specific environment variables (for blackbox fuzzers).
    • Required logging metadata (CF_TASK_NAME, CF_TASK_ARGUMENT, CF_TASK_JOB_NAME, CF_TASK_ID) to ensure logs in the subsequent uworker_main step have the correct context.

Tests performed

Executed the following command in dev:

pipenv run python butler.py preprocess --fuzzer <fuzzer> --job <job> -c <config_dir>

Successfully creates and uploads the payload and returns a valid signed URL. This signed url was later used to trigger this swarming task trough prpc, here are the logs