R1:d36d0efc3570

Add behaviors to Build Plans: hold drafts, affect buildables, warn on landing, restartable, runnable

Summary:
Depends on D20219. Ref T13258. Ref T11415. Installs sometimes have long-running builds or unimportant builds which they may not want to hold up drafts, affect buildable status, or warn during `arc land`.

Some builds have side effects (like deployment or merging) and are not idempotent. They can cause problems if restarted.

In other cases, builds are isolated and idempotent and generally safe…