Track lease usage in the schema

[alecthomas]

Similar to how we track verb-to-verb calls, we should add metadata for lease usage:

verb myVerb(Empty) Empty
  +leases user/update

Currently lease keys are completely dynamic, so we might need to introduce some sort of registration/handle system as we do for other resources, eg.

var userUpdateLease = ftl.Lease("user", "update")

//ftl:verb
func MyVerb(ctx context.Context, user User) error {
  lease, err := userUpdateLease.Acquire(ctx, user.Id)
  if err != nil {
    return err
  }
  defer lease.Release()
  // ...
}

Not sure.