[update] implement `update wait-for-commit`

This change adds the `wait-for-commit` subcommand to to the update CLI
tool. In a future CL, we'll call this in `fx ota`.

Fixed: 64593
Test: fx test -o update-lib-tests
Test: fx test -o update-integration-tests
Test: temporarily added a 40 second timer to the `wait_for_commit` fn, observed
both warning and ending log.

MULTIPLY: update-lib-tests

Change-Id: I76bb34e9cfe02d1f7aae5a75aa1c3712c7ce2234
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/458219
Commit-Queue: Zach Kirschenbaum <zkbaum@google.com>
Reviewed-by: Ben Keller <galbanum@google.com>
Testability-Review: Ben Keller <galbanum@google.com>