Add /api/update-tree-status, which is currently write-only

[matanlurey]

Towards flutter/flutter#74529.

Inserts documents into the collection tree_status_change, with the following "schema":

static Future<TreeStatusChange> create(
  FirestoreService firestore, {
  required DateTime createdOn,
  required TreeStatus status,
  required String authoredBy,
  required RepositorySlug repository,
});

To determine if the current tree should be (manually) failing, we'd run:

static Future<TreeStatusChange?> getLatest(
  FirestoreService firestore, {
  required RepositorySlug repository,
}) async {
  final docs = await firestore.query(
    metadata.collectionId,
    {'$_fieldRepository =': repository.fullName},
    limit: 1,
    orderMap: {_fieldCreateTimestamp: kQueryOrderDescending},
  );
  return docs.isEmpty ? null : TreeStatusChange.fromDocument(docs.first);
}

... however, that is not hooked up yet (we also need to create indexes first, etc).

[jtmcdole]

Can we have an optional String reason?