Translator should support HGVS splice variants

## 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. https://github.com/ga4gh/vrs/pull/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)