feat: use 'update_transforms'

[tseaver]

Update pbs_for_create, pbs_for_set_no_merge, pbs_for_set_with_merge, and pbs_for_update to match semantics expected by current versions of conformance tests:

• Rather than create separate Write.transform messages to hold field transforms, inline them as update_transforms in the main Write.update message (which will always be created now).

Copy in the current version of the conftest JSON files and verify.

Closes #217

[tseaver]

Note to reviewers: commit-by-commit is likely easier than all-at-once: the last commit is just the "copy current JSON files" bit, which doesn't really need review.