Improve `butler.py weights` script

[letitz]

Add support for:

• fuzz target weights, i.e. FuzzTargetJob db entries
  • notably including fuzz-target set
• listing FuzzerJobs entries in text format

Rework the command-line interface:

• rename data types: fuzzer_job -> fuzzer, fuzzer_jobs -> fuzzer-batch
• invert the data type and the subcommand: list fuzzer_job -> fuzzer list
• fold dump into list and add a --format={text,csv} argument to toggle between them
• extract the code setting up the CLI in butler.py for better readability

[letitz]

Thanks for the review! Addressed your comments.

[letitz]

Is there a purpose to setting these if the cron job clobbers the user set weights?

I don't think the cron job does. IIUC it clobbers the multipliers. See also https://github.com/google/clusterfuzz/issues/3937#issuecomment-2066754339