Add parameter to the /vcf endpoint to control whether VRS IDs are generated for REF alleles

[ehclark]

Added a for_ref query parameter to the /vcf endpoint that determines whether VRS IDs are generated for REF alleles. The default value is True, which is the current behavior.

The changes also include an adjustment to error handling in the VcfRegistrar._get_vrs_object() method. The VCFAnnotator in vrs-python now includes exception handling that emits a VRS_Error INFO field if an error occurs computing VRS IDs for a VCF record. So the VcfRegistrar no longer suppresses errors and logs them. They are allowed to propagate up the stack. And if no VRS object is generated by the translator, an exception is thrown.

See also #72