Translator should support HGVS splice variants

[korikuzma]

Feature Request

In #602 , we are pulling in VRS schema changes that support intron positions. This issue will add support for the translation of splice variants represented as HGVS expressions. ga4gh/vrs#679 provides examples on how to represent HGVS splice variants as VRS Relative Alleles.

Note: We will either update the current AlleleTranslator to support both VRS Allele and VRS RelativeAllele, or will create a new translator specifically for VRS RelativeAllele (RelativeAlleleTranslator).

Acceptance Critera

Given I have an HGVS splice variant (such as NM_005188.4(CBL):c.1096-1G>C),
When I run translate_from(hgvs_splice_variant_str),
Then I should get the VRS RelativeAllele normalized representation (not sure if there will be a specific RelativeAllele normalization algorithm we'll follow).

(Once the above VRS PR is merged, use examples from that)