Support Relative Activity Timing

[cartermak]

This was actually added in Aerie 1.1.0, but I'm tagging it for a v1.2.0 milestone release.

Notes from the original issue:

activity directives have definitely changed (two new fields: anchor_id and anchored_to_start). Simulated activity instances should be the same as before - nothing downstream of simulation should have changed.

• [x] Update dataclasses
• [x] Build in backwards-compatibility
• [x] Test all affected plan interfaces + build test cases (create activity, update activity, upload plan, download plan)
• [x] Document breaking changes