varianttype and alternateBases should use oneof construct in OpenAPI specs

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This has been identified in #252 but here it is as a separate issue.

I think https://swagger.io/docs/specification/data-models/oneof-anyof-allof-not/#oneof should be used for alternateBases and variantType see lines:

• https://github.com/ga4gh-beacon/specification/blob/master/beacon.yaml#L119
• https://github.com/ga4gh-beacon/specification/blob/master/beacon.yaml#L134
• https://github.com/ga4gh-beacon/specification/blob/master/beacon.yaml#L401
• https://github.com/ga4gh-beacon/specification/blob/master/beacon.yaml#L416

It is my understanding both cannot be used in queries, it should be only one of them.