Support target label for buf breaking against a registry

[emcfarlane]

Feature

Currently buf breaking --against-registry is defined as a boolean flag. If set, this will run breaking against the default label for the module in the BSR. To support feature branches, where the target branch is not the default, we need to know which label in the repository to target.

Adjust the against registry flag to support taking the target label as an optional argument. It may be possible to modify the existing flag, where if an explicit label is set via an =, or a new flag may be needed.