Add a bin/bulk export CLI tool to make debugging and profiling large exports easier

Summary:
Ref T13049. When stuff executes asynchronously on the bulk workflow it can be hard to inspect directly, and/or a pain to test because you have to go through a bunch of steps to run it again.

Make future work here easier by making export triggerable from the CLI. This makes it easy to repeat, inspect with --trace, profile with --xprofile, etc.

Test Plan:

• Ran several invalid commands, got sensible error messages.
• Ran some valid commands, got exported data.
• Used --xprofile to look at the profile for a 300MB dump of 100K tasks which took about 40 seconds to export. Nothing jumped out as sketchy to me -- CustomField wrangling is a little slow but most of the time looked like it was being spent legitimately.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13049

Differential Revision: https://secure.phabricator.com/D18965