Remove `ga4gh:` prefix from VCF annotations?

Currently, the VRS VCF annotator can be used to add full VRS IDs for all REF and ALT alleles to VCF records, e.g. (you'll probably need to scroll right):

```
chr1 783006 . A G 50 PASS platforms=4;platformnames=PacBio,Illumina,10X,CG;datasets=4;datasetnames=CCS15kb_20kb,HiSeqPE300x,10XChromiumLR,CGnormal;VRS_Allele_IDs=ga4gh:VA.dwwiZdvVtfAmomu0OBsiHue1O-bw5SpG,ga4gh:VA.MiasxyXMXtOpsZgGelL3c4QgtflCNLHD;VRS_Starts=783005,783005;VRS_Ends=783006,783006;VRS_States=A,G GT:PS:DP:ADALL:AD:GQ 1/1:.:652:16,234:0,82:312
```

These include `ga4gh:` prefixes. In a file with a lot of rows, this could add a small but nontrivial amount of memory; can we infer (or maybe state explicitly in the corresponding header description) the prefix and leave it off to conserve space?

Similarly, can we leave the `VA.` prefix? Will VCF attributions with this tool always produce alleles?