VariantWalker / VariantContext doesn't validate variants at parse-time

[jonn-smith]

VariantWalker and VariantContext do not do variant validation at parse-time.

This causes awkward errors on invalid files like the one found here:
https://gatkforums.broadinstitute.org/gatk/discussion/23809/oncotator-for-build-hg38

It would be best if there was a way to properly validate the variants before parsing them. ValidateVariants currently doesn't properly work when given default options (#5862).

When #5862 is fixed, this may be ignored - I think validating at run-time when iterating over variants may add too much overhead.