(Investigate) Support for mateName for fusion

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Description

Investigate support for adding mateName mateName attribute to BeaconAlleleRequest, to allow for the query of fusion events using the "BND" variantType

DoD (Definition of Done)

mateName key along with necessary functionality and changes added to code base as Proof of Concept.

Testing

Unit Testing

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For tracking purposes, started this issue: ga4gh-beacon/specification#277

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Datasets that could be used for testing:

• https://github.com/hall-lab/svtyper/blob/master/tests/data/example.vcf (example)
• https://github.com/AstraZeneca-NGS/publication_data/ from this paper from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5382922/