Add callback support for VCF annotator

[jsstevenson]

Second, this may be an opportunity to improve the relationship between AnyVar and vrs-python. The AnyVar VcfRegistrar extends VCFAnnotator and overrides the internal _get_vrs_object() method. The only reason for doing so is to integrate with the AnyVar object store. But then AnyVar does not actually re-implement all the vrs-python functionality, which means there will be continual drift between the two implementations. It seems like whether you annotate a VCF with VRS IDs directly in vrs-python or via AnyVar that the behavior and result should be the same. That would make the tools easier to adopt and use. So perhaps a re-design of the vrs-python VCFAnnotator to provide a listener or callback. For example:

class VCFAnnotator:
  # ...existing implementation...

  def on_vrs_object(self, vcf_coords: str, vrs_object: VrsObject) -> bool:
    """Called for each calculated VRS object prior to it being added to the VCF file.
    :param vcf_coords: The VCF coordinates of the VRS object
    :param vrs_object: The computed VRS object
    :return:  True if the VRS object should be added to the output VCF, False otherwise
    """
    return True

# And in AnyVar
class VcfRegistrar(VCFAnnotator):
  def on_vrs_object(self, vcf_coords: str, vrs_object: VrsObject) -> bool:
    self.av.put_object(vrs_object)
    return True

originally posted by @ehclark in #470 (comment)