[Integrity] Write helper to collectively return missing dependencies from must_get calls

[jost-s]

A validation fn could look like this:

let entry = must_get_entry()?; // 1
check_if_entry_is_cool();
let action = must_get_action()?; // 2
check_if_action_is_cool();

If neither entry nor action are available locally, app validation will run once and get to 1, then return UnresolvedDependencies, wait 10 seconds, run again and get to 2, return UnresolvedDependencies, wait 10 seconds, run again and complete.

Instead an app dev could write:

let maybe_entry = must_get_entry();
let maybe_action = must_get_action();
let unresolved_dependencies = build_unresolved_dependencies_vector([maybe_entry, maybe_action]);
if unresolved_dependencies.len() > 1 {
  ValidateCallbackResult::UnresolvedDependencies(UnresolvedDependencies::Hashes(unresolved_dependencies))
} else {
  ValidateCallbackResult::Valid
}

AC:

• Write a helper to handle the results of multiple must_gets, bundling missing hashes or agent activity together
• possible interface:

let (entry, action) = resolve!(
  must_get_entry(hash1),
  must_get_action(hash2),
);